Config Tests use System Config instead of Test Config

Created by: NickDaly

When running nosetests to test the system, the plinth/tests/test_cfg.py tests use the system-wide config file (/etc/plinth/config) instead of the test config file (in plinth/tests/data). This leads to inaccurate testing.

This issue is easy to reproduce when running Plinth 0.6 tests when Plinth 0.5 is installed on the system, and will prevent the package from building (therefore, it's impossible to build the Plinth 0.6 package when Plinth < 0.6 is installed on the system).