789fba66 broke running tests locally

How Postorius' tests are setup isn't very nice.
Currently just running tox locally fails, because all the commands are defined for jobs that include coverage. So currently you can't run tests without coverage, making the addition of coverage to the environment names redundant...

I propose changing the tests, so they run without coverage. These tests will also process faster than coverage jobs. In addition to that we add a job that runs coverage for a given python/django version.

We could then also publish the coverage reports to gitlab pages, so people that want to contribute tests, can easily see online what parts still need testing. AFAIK there is no other free alternative that runs coverage and serves the reports online. Here is a tutorial on how to do so.