MediaWiki/tests/phpunit/includes/skins/SkinTest.php

<?php
class SkinTest extends MediaWikiTestCase {

	/**
	 * @covers Skin::getDefaultModules
	 */
	public function testGetDefaultModules() {
		$skin = $this->getMockBuilder( Skin::class )
			->setMethods( [ 'outputPage', 'setupSkinUserCss' ] )
			->getMock();

		$modules = $skin->getDefaultModules();
		$this->assertTrue( isset( $modules['core'] ), 'core key is set by default' );
		$this->assertTrue( isset( $modules['styles'] ), 'style key is set by default' );
	}
}