Completer test should not depend on actual cfg file

If you enable a preview service in your real config file, you'll
get test failures.  Similar thing for plugins.

The fix is to point it at an empty cfg file so that we don't load
any real config files for unit tests.
Merge branch 'completer-test-preview' into develop

* completer-test-preview:
  Completer test should not depend on actual cfg file