Add pytest fixture for checking test artifacts

Add pytest fixture for checking test artifacts

Prior to introducing the pytest runner, clean.sh files were used as a
list of files that the test is expected to leave around as artifacts and
check that no extra files were created.

With the pytest runner, those scripts are no longer used, but the
ability to detect extraneous files is still useful. Add a new
"extra_artifacts" mark which can be used for the same purpose.

(cherry picked from commit 3a9f4edddcfba0d4d303f0ae6873a9f0c07966d7)