Unit tests for https://github.com/WordPress/wordpress-develop/pull/6720 -- the `wp_salt()` function uses a static variable so it's not possible to include these in the test suite.

wpSalt.php

<?php

/**
 * @group pluggable
 *
 * @covers ::wp_salt
 */
class Tests_Pluggable_wp_salt extends WP_UnitTestCase {
	/**
	 * Test that the signature options of wp_salt() are primed in a single database call (single site).
	 *
	 * As the slats can be defined via the `wp-config.php` file, this test is somewhat
	 * flaky and can not test the priming of the caches if the constant is properly
	 * defined.
	 *
	 * As such it is slightly reversed in that it tests for the absence of a database
	 * query rather than the presence of a cache priming query.
	 *
	 * @dataProvider data_wp_salt_cache_priming
	 *
	 * @ticket 59871
	 *
	 * @group ms-excluded
	 */
	public function test_wp_salt_cache_priming_for_single_site( $salt_scheme ) {
		// Skip the test if the constant is defined and not the placeholder string.
		if (
			defined( strtoupper( $salt_scheme ) )
			&& 'put your unique phrase here' !== constant( strtoupper( $salt_scheme ) )
		) {
			$this->markTestSkipped( 'The constant is defined and not the placeholder string.' );
		}

		// Set the options directly to ensure the option is populated.
		update_site_option( "{$salt_scheme}_key", wp_generate_password() );
		update_site_option( "{$salt_scheme}_salt", wp_generate_password() );

		global $wpdb;
		$unexpected_database_query_key  = "SELECT option_value FROM {$wpdb->options} WHERE option_name = '{$salt_scheme}_key' LIMIT 1";
		$unexpected_database_query_salt = "SELECT option_value FROM {$wpdb->options} WHERE option_name = '{$salt_scheme}_salt' LIMIT 1";

		// Clear the options cache to ensure the query is not cached.
		wp_cache_delete( "{$salt_scheme}_key", 'options' );
		wp_cache_delete( "{$salt_scheme}_salt", 'options' );

		// Cache database queries via the query hook.
		$queries = array();
		add_filter(
			'query',
			function ( $query ) use ( &$queries ) {
				$queries[] = $query;
				return $query;
			}
		);
		// Call the function to prime the cache.
		wp_salt( $salt_scheme );

		// Check that the queries cache does not contain the unexpected queries.
		$this->assertNotContains( $unexpected_database_query_key, $queries );
		$this->assertNotContains( $unexpected_database_query_salt, $queries );
	}

	/**
	 * Test that the signature options of wp_salt() are primed in a single database call (multisite).
	 *
	 * See test above for limitations of this test.
	 *
	 * @dataProvider data_wp_salt_cache_priming
	 *
	 * @ticket 59871
	 *
	 * @group ms-required
	 */
	public function test_wp_salt_cache_priming_for_multisite( $salt_scheme ) {
		// Skip the test if the constant is defined and not the placeholder string.
		if (
			defined( strtoupper( $salt_scheme ) )
			&& 'put your unique phrase here' !== constant( strtoupper( $salt_scheme ) )
		) {
			$this->markTestSkipped( 'The constant is defined and not the placeholder string.' );
		}

		// Set the options directly to ensure the option is populated.
		update_site_option( "{$salt_scheme}_key", wp_generate_password() );
		update_site_option( "{$salt_scheme}_salt", wp_generate_password() );

		global $wpdb;
		$network_id                     = (int) get_current_network_id();
		$unexpected_database_query_key  = "SELECT meta_value FROM {$wpdb->sitemeta} WHERE meta_key = '{$salt_scheme}_key' AND site_id = {$network_id}";
		$unexpected_database_query_salt = "SELECT meta_value FROM {$wpdb->sitemeta} WHERE meta_key = '{$salt_scheme}_salt' AND site_id = {$network_id}";

		// Clear the options cache to ensure the query is not cached.
		wp_cache_delete( "{$network_id}:{$salt_scheme}_key", 'site-options' );
		wp_cache_delete( "{$network_id}:{$salt_scheme}_salt", 'site-options' );

		// Cache database queries via the query hook.
		$queries = array();
		add_filter(
			'query',
			function ( $query ) use ( &$queries ) {
				$queries[] = $query;
				return $query;
			}
		);
		// Call the function to prime the cache.
		wp_salt( $salt_scheme );

		// Check that the queries cache does not contain the unexpected queries.
		$this->assertNotContains( $unexpected_database_query_key, $queries );
		$this->assertNotContains( $unexpected_database_query_salt, $queries );
	}

	public function data_wp_salt_cache_priming() {
		return array(
			array( 'nonce' ),
			array( 'auth' ),
			array( 'secure_auth' ),
			array( 'logged_in' ),
		);
	}
}