tox: Allow passing positional arguments

Allows us to pass on arguments to run_tests and pytest after -- when
executing tox.
E.g.: To run all tests verbose in a test class:
  tox -- -v tests/test_project.py::ReviewableBranchTests

Signed-off-by: Daniel Kutik <daniel.kutik@lavawerk.com>
Change-Id: Ibd78856c6d4053c769f3d0b6130ebc8145275f78
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/353176
Reviewed-by: Mike Frysinger <vapier@google.com>
diff --git a/tox.ini b/tox.ini
index feb328e..9a8b3fc 100644
--- a/tox.ini
+++ b/tox.ini
@@ -29,7 +29,7 @@
 deps =
     pytest
     pytest-timeout
-commands = {envpython} run_tests
+commands = {envpython} run_tests {posargs}
 setenv =
     GIT_AUTHOR_NAME = Repo test author
     GIT_COMMITTER_NAME = Repo test committer