[PATCH 2 of 2] tests: replace yet more calls to `python` with $PYTHON

[Jun Wu]

Excerpts from Augie Fackler's message of 2017-06-29 22:08:41 -0400:
> Yeah.
> 
> We also need a way to land patches out of phabricator, which I think might
> take some doing.

Phabricator has "RAW patch" feature. Might be just one API call and it
returns plain text patches suitable for "hg import". I can make that work
too.