Load optional apps when testing

Consider having an app that is best described as optional, but includes some tests. For instance, imagine adding a test to ``filebrowser_safe`` ;-) Then you could try to test it using:
```bash
./manage.py test filebrowser_safe
```
but you'd get just a lookup error. Moreover, disabling optional apps might let the tests succeed, where in reality there would be some integration problem.

There surely was / is some argument for excluding optional apps from testing, so the change may be disputable, but I've hit it a few of times already, so would like to have a clear view on the matter.