Fix run_test_module logic (#53884)
Summary:
First argument is either file name or test module name, but key to `CUSTOM_HANDLERS` is test module name.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53884
Test Plan: Run `python3 run_test.py -i distributed/test_distributed_spawn.py`
Reviewed By: janeyx99
Differential Revision: D27006164
Pulled By: malfet
fbshipit-source-id: f30b42856cd2754e5981c1c69618f84e392c986a
