WIP: test_does_not_add_new_test_files: try git ls-tree first

... and then fallback on existing glob.iglob(**) if git is not found or fails for any reason (e.g.: tarball)

In other words, look at test files that have actually been added to git and not at random files that just happen to be around.

Having to git clean temporary test files before running the whole test suite is not convenient.