Test that plugin called `load_plugin_textdomain()` correctly.

PluginTest.php

<?php

namespace GAA\CorpManager;

use WP_UnitTestCase;

class PluginTest extends WP_UnitTestCase {

	private $domain;
	private $mofile;

	/**
	 * @covers GAA\CorpManager\Plugin::load_plugin_textdomain
	 */
	public function test_load_plugin_textdomain() {
		// Plugin-specific values.
		$text_domain   = 'corp-manager';
		$languages_dir = 'languages/';
		$load_callable = [ $GLOBALS[ 'corp_manager_plugin' ], 'load_plugin_textdomain' ];
		$locale        = $this->_locale_cb();

		// Check the plugin method that loads the text domain is hooked into the right filter.
		$this->assertNotFalse( has_action( 'plugins_loaded', $load_callable ) );

		// Change the locale to Spanish.
		add_filter( 'locale', [ $this, '_locale_cb' ] );

		// Hook in to this action, so we can extract what the domain and mofile path will be the next time
		// a text domain is loaded.
		add_action( 'override_load_textdomain', array( $this, '_override_load_textdomain_cb' ), 10, 3 );

		// Call the textdomain loading function.
		call_user_func( $load_callable );

		// Remove the hack as we don't care about any future textdomain loading calls.
		remove_action( 'override_load_textdomain', array( $this, '_override_load_textdomain_cb' ), 10, 3 );

		// Check that the loaded domain name is what we think it should be.
		$this->assertSame( $text_domain, $this->domain );

		// Check the path to the loaded mofile is correct.
		// Since `load_plugin_textdomain()` will prefix the relative path with the path to the test directory path
		// i.e. wordpress-develop/wp-content/plugins, regardless of where your plugin is located, then we just
		// check the string contains the bit we know it should have.
		$this->assertStringEndsWith( "/{$text_domain}/{$languages_dir}{$text_domain}-{$locale}.mo", $this->mofile );

		// Revert the locale change.
		remove_filter( 'locale', [ $this, '_locale_cb' ] );
	}

	public function _override_load_textdomain_cb( $override, $domain, $mofile ) {
		$this->domain = $domain;
		$this->mofile = $mofile;

		return $override; // or simply true?
	}

	public function _locale_cb() {
		return 'es_ES';
	}
}

Huge hat-tip to @swissspidy, as most of the code is his!