Google tag manager tests are dependent on external configuration

[This test](https://github.com/mozilla/experimenter/blob/master/app/experimenter/base/tests/test_views.py#L8) will fail if the environment variable `USE_GOOGLE_ANALYTICS=false` is set in your local settings, which should not be the case.