Start on PHPUnit tests

Why:
* The WikimediaMessages extension currently has no tests.
* This is not ideal, considering that the extension is deployed on
  WMF production and can be run on every page load (such as
  through the ::onBeforePageDisplay hook).
* Testing the extension makes the code easier to maintain
  and more reliable to regressions.

Why:
* Add some unit tests that cover a few easy to cover areas of
  Hooks.php.
* This is done separately to make it easier to see the coverage
  change in future patches.

Bug: T315774
Change-Id: I916668ae0375e8668b09a16111d83bf2e2feee40

tests/phpunit/unit/HooksTest.php

@@ -0,0 +1,60 @@
+<?php
+
+namespace MediaWiki\Extension\WikimediaMessages\Tests\Unit;
+
+use MediaWiki\Config\HashConfig;
+use MediaWiki\Extension\WikimediaMessages\Hooks;
+use MediaWiki\Output\OutputPage;
+use MediaWiki\Tests\Unit\MockServiceDependenciesTrait;
+use MediaWikiUnitTestCase;
+use Skin;
+use Title;
+
+/**
+ * @covers \MediaWiki\Extension\WikimediaMessages\Hooks
+ */
+class HooksTest extends MediaWikiUnitTestCase {
+
+	use MockServiceDependenciesTrait;
+
+	/** @dataProvider provideOnBeforePageDisplay */
+	public function testOnBeforePageDisplay( $wikimediaStyleSkins, $currentSkin, $expectedModuleStyles ) {
+		/** @var Hooks $hooks */
+		$hooks = $this->newServiceInstance( Hooks::class, [] );
+		// Make Skin::getSkinName return $currentSkin
+		$skin = $this->createMock( Skin::class );
+		$skin->method( 'getSkinName' )
+			->willReturn( $currentSkin );
+		// Make OutputPage::getConfig return a HashConfig with wgWikimediaStylesSkins defined.
+		$out = $this->createMock( OutputPage::class );
+		$out->method( 'getConfig' )
+			->willReturn( new HashConfig( [ 'WikimediaStylesSkins' => $wikimediaStyleSkins ] ) );
+		// Assert that OutputPage::addModuleStyles is called with $expectedModuleStyles
+		$out->expects( $expectedModuleStyles !== [] ? $this->atLeastOnce() : $this->never() )
+			->method( 'addModuleStyles' )
+			->with( $expectedModuleStyles );
+		// Call ::onBeforePageDisplay with the mocks we just created.
+		$hooks->onBeforePageDisplay( $out, $skin );
+	}
+
+	public static function provideOnBeforePageDisplay() {
+		return [
+			'No skins are defined in config' => [ [], 'vector', [] ],
+			'Skin is not included in config' => [ [ 'vector' ], 'monobook', [] ],
+			'Skin is included in config' => [ [ 'vector' ], 'vector', [ 'ext.wikimediamessages.styles' ] ],
+		];
+	}
+
+	public function testOnSkinCopyrightFooterForHistoryType() {
+		// Calls ::onSkinCopyrightFooter with the $type argument as history and expect that it just returns without
+		// doing anything else. This is checked by ensuring the $msg and $link provided by reference is not modified.
+		/** @var Hooks $hooks */
+		$hooks = $this->newServiceInstance( Hooks::class, [] );
+		$msg = 'msg';
+		$link = 'link';
+		$hooks->onSkinCopyrightFooter( $this->createMock( Title::class ), 'history', $msg, $link );
+		// Check that the $msg and $link variables provided by reference were not changed.
+		$this->assertEquals( 'msg', $msg );
+		$this->assertEquals( 'link', $link );
+	}
+}