Enhancement/updated tests

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

It should not always be a 404, it depends. If the Request already contains an error, a 400 should be returned, if the request is targeting an object which is not existing, it's a 404. There are also other cases. And extending the HttpNotFoundException is also not a solution, since this is makes only sense in the REST-context.