We were running `collectstatic` as part of our test setup for djblets, but this wasn't actually necessary, and doing so prevents usage of the `pytest-xdist` plugin which is super nice for development. This change removes it. While in there, I also changed our `pytest_report_header` hook to print the version of djblets and *not* print the version of django, since the django version is already printed by the pytest-django plugin. Testing Done: Ran unit tests, both with xdist and normally.