Enhancement/updated tests

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

The `MediaType::$name` should not be changed. 

Its a good idea only use one. I would prefer to use the config to save some SQL requests and then we save to the `fileVersion` entity the mediaType as string e.g. `image` and so we need to ids. For this we also need to create an update script for old versions. Please create an Issue for that.