Fix GhostFieldAccessTrait tests

Change-Id: Id03febbcb9d250a649eb03006f4984d0fb38d8e5

tests/phpunit/includes/libs/serialization/SerializationTestUtils.php

@@ -62,6 +62,9 @@ class SerializationTestUtils {
 		callable $serializer,
 		callable $deserializer
 	) {
+		if ( !is_dir( $serializedDataPath ) ) {
+			throw new InvalidArgumentException( "{$serializedDataPath} does not exist" );
+		}
 		$this->serializedDataPath = $serializedDataPath;
 		$this->ext = $ext;
 		$this->serializer = $serializer;

tests/phpunit/unit/includes/libs/GhostFieldAccessTraitTest.php

@@ -12,23 +12,14 @@ use Wikimedia\Tests\SerializationTestUtils;
  */
 class GhostFieldAccessTraitTest extends MediaWikiUnitTestCase {
 
-	private $serializationTestUtils;
-
 	protected function setUp() : void {
 		parent::setUp();
-		// Not using the trait since we only need deserialization tests.
-		$this->serializationTestUtils = new SerializationTestUtils(
-			__DIR__ . '/../../data/GhostFieldAccess',
-			[],
-			'serialized',
-			'serialize',
-			'unserialize'
-		);
 	}
 
 	private function provideUnserializedInstances( string $testCaseName ) {
+		// Not using the trait since we only need deserialization tests.
 		$serializationTestUtils = new SerializationTestUtils(
-			__DIR__ . '/../../data/GhostFieldAccess',
+			__DIR__ . '/../../../data/GhostFieldAccess',
 			[],
 			'serialized',
 			'serialize',