tests: Add a testdatadir fixture
tests: Add a testdatadir fixture

This just returns a path to the 'data' directory for the test using the
fixture.

For example, a test in ideascube/serveradmin/tests/test_catalog.py would
get testdatadir pointing to ideascube/serveradmin/tests/data/

ideascube/ideascube,ideascube/ideascube,ideascube/ideascube,ideascube/ideascube