stop test from failing the second time it is run

This test references `__file__`, which, the first time it is run ends with a .py, however, the second time it is run, it will be a .pyc (since we no longer run test_xacro.py directly). This (somewhat hacky) fix will make sure we always use the .py filename.

If anyone has a better way to do this, I'm all ears. If not, I'll also open a hydro pull request once this is merged.