tests, preferences: Use $this-> instead of self:: when creating TD

In core, Test Doubles (TDs) are almost always created like that (using $this->),
lets stick to a unified way of things.

Change-Id: Ic8e62ccec19f136bc4cc6b27deefb43fd789ef07

tests/phpunit/unit/includes/preferences/FiltersTest.php

@@ -114,7 +114,7 @@ class FiltersTest extends \MediaWikiUnitTestCase {
 			'Baz' => 3,
 		];
 		$flipped = array_flip( $userMapping );
-		$idLookup = self::getMockBuilder( CentralIdLookup::class )
+		$idLookup = $this->getMockBuilder( CentralIdLookup::class )
 			->disableOriginalConstructor()
 			->setMethods( [ 'centralIdsFromNames', 'namesFromCentralIds' ] )
 			->getMockForAbstractClass();