Bazel operates by simply running the python scripts given to it in `py_test`. If the script doesn't invoke pytest on itself in the `if _name__ == "__main__"` snippet, no tests will be ran, and the script will pass. This has led to several tests (indeed, some are fixed in this PR) that, despite having been written, have never ran in CI. This PR adds a lint check to check all `py_test` sources for the presence of `if _name__ == "__main__"` snippet, and will fail CI if there are any detected without it. This system is only enabled for libraries right now (tune, train, air, rllib), but it could be trivially extended to other modules if approved.
