phpunit: Fix OutputPage::__construct warning in SkinTemplateTest

https://integration.wikimedia.org/ci/job/mediawiki-core-code-coverage/2562/console
> Deprecated: Use of OutputPage::__construct was deprecated in MediaWiki 1.18.
> .. SkinTemplateTest->getMockOutputPage() ..

Change-Id: I7f44ffecc40b9d972839554361660e5b38ce3aef

tests/phpunit/includes/skins/SkinTemplateTest.php

@@ -44,7 +44,9 @@ class SkinTemplateTest extends MediaWikiTestCase {
 	 * @return PHPUnit_Framework_MockObject_MockObject|OutputPage
 	 */
 	private function getMockOutputPage( $isSyndicated, $html ) {
-		$mock = $this->getMock( OutputPage::class );
+		$mock = $this->getMockBuilder( OutputPage::class )
+			->disableOriginalConstructor()
+			->getMock();
 		$mock->expects( $this->once() )
 			->method( 'isSyndicated' )
 			->will( $this->returnValue( $isSyndicated ) );