tests: drop old unittest.main logic

We use pytest now which doesn't need this boilerplate.

Change-Id: Ib71d90b3f1669897814ee768927b5b595ca8d789
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/354355


Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Tested-by: Mike Frysinger <vapier@google.com>