Use MapperTestUtility from the Appframework instead of importing the class

[oparoz]

The MapperTestUtility class is already part of the server, so let's use that instead of importing our own:

https://github.com/nextcloud/server/blob/e991b23d9d74a7d8af9d7f824a5bbeaf1e629604/tests/lib/AppFramework/Db/MapperTestUtility.php

[imjalpreet]

@oparoz Earlier I had done this but it was giving the following error:

PHP Fatal error: Class 'Test\AppFramework\Db\MapperTestUtility' not found in /var/www/GSoC/apps/mediametadata/tests/unit/Services/RetrieveMetadataTest.php on line 22