Enhancement/updated tests

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

So now we retrieve the MediaType from the database by its *name*. This makes the code portable -- we should never hard-code dynamic IDs (not even in functional tests).

But will this cause any issues? f.e. is the `MediaType::$name` property mutable? /cc @alexander-schranz @danrot