gitlab-ci: move sdist test run to new fedora job

A full run of the test suite takes quite a bit of time.  This removes one
of the 3 runs from the main 'tests' job, and puts it into the Fedora job.
That test run is mostly to make sure the setup.py and source tarball are
correctly, so that doesn't affect merge requests very often.

This also tests `pip install --user`, which was not really being tested
before.