The purpose of these files is explained here: http://jeremypry.com/plugin-theme-development-wordpress-trunk/

LocalValetDriver.php

<?php

// PR #237 broke Valet+ custom drivers, this is a workaround.
if ( ! class_exists( 'WordPressValetDriver' ) ) {
	$driverPath = realpath( $_SERVER['HOME'] ) . '/.composer/vendor/weprovide/valet-plus/cli/drivers/WordPressValetDriver.php';
	if ( ! is_readable( $driverPath ) ) {
		throw new Exception( 'Unable to load the WordPress Valet Driver.' );
	}

	require_once $driverPath;
	unset( $driverPath );
}

/**
 * Site-specific valet driver.
 *
 * @author Jeremy Pry
 */
class LocalValetDriver extends WordPressValetDriver {

	/**
	 * The root of the site.
	 *
	 * @var string
	 */
	protected $siteRoot;

	/**
	 * Determine if the driver serves the request.
	 *
	 * Always return true, since this is a local driver.
	 *
	 * @param string $sitePath The root path to the current site.
	 * @param string $siteName The name of the current site.
	 * @param string $uri      The relative URI to serve.
	 *
	 * @return bool
	 */
	public function serves( $sitePath, $siteName, $uri ) {
		$this->siteRoot = $sitePath;
		return true;
	}

	/**
	 * Get the fully resolved path to the application's front controller.
	 *
	 * @param string $sitePath The root path to the current site.
	 * @param string $siteName The name of the current site.
	 * @param string $uri      The relative URI to serve.
	 *
	 * @return string
	 */
	public function frontControllerPath( $sitePath, $siteName, $uri ) {
		// Allow serving random PHP files.
		if ( 'php' === substr( $uri, -3 ) && file_exists( "{$sitePath}{$uri}" ) ) {
			return "{$sitePath}{$uri}";
		}

		if ( preg_match( '#/(?:wp-)?content#', $uri ) ) {
			$sitePath .= '/content';
		} else {
			$sitePath .= '/wp/build';
		}

		return parent::frontControllerPath( $sitePath, $siteName, $uri );
	}

	/**
	 * Determine if this is a static file.
	 *
	 * This first tries to find the file at the actual URI. It will then
	 * fall back to the WordPress build directory. If it still doesn't
	 * match a known file, then return false.
	 *
	 * @param string $sitePath The root path to the current site.
	 * @param string $siteName The name of the current site.
	 * @param string $uri      The relative URI to serve.
	 *
	 * @return string|false
	 */
	public function isStaticFile( $sitePath, $siteName, $uri ) {
		if ( $this->isActualFile( $staticFilePath = $sitePath . $uri ) ) {
			return $staticFilePath;
		} elseif ( $this->isActualFile( $staticFilePath = "{$sitePath}/wp/build{$uri}" ) ) {
			return $staticFilePath;
		}

		return false;
	}

	/**
	 * Load server environment variables if available.
	 *
	 * @param string $sitePath The root path to the current site.
	 * @param string $siteName The name of the current site.
	 */
	protected function loadServerEnvironmentVariables( $sitePath, $siteName ) {
		if ( file_exists( $varFilePath = $this->siteRoot . '/.env.valet' ) ) {
			$variables = include $varFilePath;
		} elseif ( file_exists( $varFilePath = $sitePath . '/.env.valet' ) ) {
			$variables = include $varFilePath;
		} else {
			return;
		}

		if ( ! isset( $variables[ $siteName ] ) ) {
			return;
		}
		foreach ( $variables[ $siteName ] as $key => $value ) {
			$_SERVER[ $key ] = $value;
		}
	}
}

wp-cli.yml

path: wp/build/

wp-config.php

<?php
/**
 * The base configuration for WordPress.
 *
 * All extraneous comments (besides this one) removed for brevity.
 */

// ** MySQL settings ** //
define( 'DB_NAME', '<PROJECT NAME>' ); // Update this value based on the project
define( 'DB_USER', '<PROJECT DB USER>' );
define( 'DB_PASSWORD', '<PROJECT DB PASSWORD>' );
define( 'DB_HOST', 'localhost' );
define( 'DB_CHARSET', 'utf8' );
define( 'DB_COLLATE', '' );

/** Authentication Unique Keys and Salts. */
define( 'AUTH_KEY',         'put your unique phrase here' );
define( 'SECURE_AUTH_KEY',  'put your unique phrase here' );
define( 'LOGGED_IN_KEY',    'put your unique phrase here' );
define( 'NONCE_KEY',        'put your unique phrase here' );
define( 'AUTH_SALT',        'put your unique phrase here' );
define( 'SECURE_AUTH_SALT', 'put your unique phrase here' );
define( 'LOGGED_IN_SALT',   'put your unique phrase here' );
define( 'NONCE_SALT',       'put your unique phrase here' );

$table_prefix = 'wp_';

define( 'WP_DEBUG', true );
define( 'WP_DEBUG_DISPLAY', false );
define( 'WP_DEBUG_LOG', true );
define( 'SCRIPT_DEBUG', true );
define( 'JETPACK_DEV_DEBUG', true );
define( 'JETPACK_STAGING_MODE', true );
define( 'WP_POST_REVISIONS', 5 );
define( 'WCS_DEBUG', true );

// Custom content directory
define( 'WP_CONTENT_DIR', dirname( __DIR__ ) . '/content' );
define( 'WP_CONTENT_URL', 'https://<PROJECT FOLDER NAME>.test/content' );

/* That's all, stop editing! Happy blogging. */

/** Absolute path to the WordPress directory. */
if ( ! defined( 'ABSPATH' ) ) {
	define( 'ABSPATH', __DIR__ . '/build/' );
}

/** Sets up WordPress vars and included files. */
require_once ABSPATH . 'wp-settings.php';

wp-tests-config.php

<?php
/**
 * The base configuration for WordPress tests.
 *
 * All extraneous comments (besides this one) removed for brevity. See wp-tests-config-sample.php.
 */

/* Path to the WordPress codebase you'd like to test. Add a forward slash in the end. */
if ( defined( 'WP_RUN_CORE_TESTS' ) && WP_RUN_CORE_TESTS ) {
	define( 'ABSPATH', dirname( __FILE__ ) . '/build/' );
} else {
	define( 'ABSPATH', dirname( __FILE__ ) . '/src/' );
}

define( 'WP_DEFAULT_THEME', 'default' );
define( 'WP_TESTS_MULTISITE', false );
define( 'WP_TESTS_FORCE_KNOWN_BUGS', false );
define( 'WP_DEBUG', true );

// ** MySQL settings ** //
// WARNING WARNING WARNING!
// These tests will DROP ALL TABLES in the database with the prefix named below.
// DO NOT use a production database or one that is shared with something else.
define( 'DB_NAME', 'wp_test_db' ); // Different from the DB found in wp-config.php!!!
define( 'DB_USER', '<TESTS DB USER>' );
define( 'DB_PASSWORD', '<TESTS DB PASSWORD>' );
define( 'DB_HOST', 'localhost' );
define( 'DB_CHARSET', 'utf8' );
define( 'DB_COLLATE', '' );

define( 'AUTH_KEY',         'put your unique phrase here' );
define( 'SECURE_AUTH_KEY',  'put your unique phrase here' );
define( 'LOGGED_IN_KEY',    'put your unique phrase here' );
define( 'NONCE_KEY',        'put your unique phrase here' );
define( 'AUTH_SALT',        'put your unique phrase here' );
define( 'SECURE_AUTH_SALT', 'put your unique phrase here' );
define( 'LOGGED_IN_SALT',   'put your unique phrase here' );
define( 'NONCE_SALT',       'put your unique phrase here' );

$table_prefix = 'wptests_';

// Custom content directory
define( 'WP_CONTENT_DIR', dirname( __DIR__ ) . '/content' );
define( 'WP_CONTENT_URL', 'https://<PROJECT FOLDER NAME>.test/content' );

define( 'WP_TESTS_DOMAIN', 'example.org' );
define( 'WP_TESTS_EMAIL', 'admin@example.org' );
define( 'WP_TESTS_TITLE', 'Test Blog' );
define( 'WP_PHP_BINARY', 'php' );
define( 'WPLANG', '' );