Every tests/test_*.py file provides an
if __name__ == "__main__":
unittest.main(...)
entrypoint so it can be run directly, except test_ssti.py and
test_xpath.py. Running either of those directly produced no output and
executed none of its tests; they only ran under `unittest discover`.
Add the missing entrypoint to both so they behave like the rest of the
suite. No test logic changed (62 and 48 tests respectively still pass).
