Restore support for user-defined job submission methods.

As documented in the user guide but probably lost quite some time ago, we're supposed to be able to add new job submission methods (or derive from the existing ones) in a ```python``` sub-dir of the suite definition directory.

This restores the ```python``` sub-directory to ```sys.path```, and adds a test.

@matthewrmshin - please review.

(background: I may need this in order to make a test for #880)