tests: unify fixture() helper with Path constant

Change-Id: I63751042391f5cc3e06af7067bc83d67bd0716dc
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/569441
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

tests/test_wrapper.py

@@ -29,11 +29,6 @@ import main
 import wrapper
 
 
-def fixture(*paths):
-    """Return a path relative to tests/fixtures."""
-    return os.path.join(os.path.dirname(__file__), "fixtures", *paths)
-
-
 class RepoWrapperTestCase(unittest.TestCase):
     """TestCase for the wrapper module."""