Enhancement/updated tests

- Migrated the tests to use `SuluTestCase`.
- Removed the hard coded IDs for MediaTypes << potential BC break.

I removed the exception handling so I could actually debug the code, so this should not be here - but as it is: Why do we wrap these exceptions?

Symfony will automatically serialize the exceptions to JSON format anyway, and these exceptions contain stack traces and are useful, so I would prefer to keep it simple and let Symfony/FOSRest do its thing. wdyt? @alexander-schranz @danrot