Use mock for database-based tests

[zesch]

Tests that depend on a database can only be run in the UKP environment.
We should replace those with a mocked version

Would also solve #6

[logological]

@zesch: Do you have any further details on how this should be implemented?

[logological]

@reckart suggests creating and checking in a small dummy GermaNet that can be used for testing purposes. Judging from the GermaNet file structure, I think this is doable but might take a while, since there are a whole bunch of interrelated files. (Or alternatively, it might be faster to use the GernEdiT tool that comes with GermaNet to create a new mini-GermaNet from scratch.)