Fix broken tests and improve database tests

[dwitvliet]

Make sure that database tests function independently of the actual data, meaning that all tests should still work if the data changes (e.g. see https://github.com/catmaid/CATMAID/blob/master/django/applications/catmaid/tests/test_common_apis.py). Also, the database tests should function without the need to manually create and populate a test database during installation; it should all be done automatically during testing.

• [x] Fix broken tests.
• [x] Make tests independent of data.
• [x] Add pre- and postamble to automatically populate database on tests.

[d2fong]

working on fixing broken tests

[d2fong]

@dwitvliet Can we close this issue or is there more to do here?