Bug 968794 - Test test_settings_do_not_track fails intermittently on dev...

...ice
The test was moving to fast
I have just inverse the asserts, because here:
https://github.com/mozilla-b2g/gaia/blob/master/tests/python/gaia-ui-tests/gaiatest/apps/settings/regions/do_not_track.py#L23
we are waiting for the setting  to get checked and only then we can assert if it's enabled.