python tests: use py shebang instead of $(PYTHON)

I'm going to make the external tests manually launchable. For that I first had
an error message if $(PYTHON) was empty. But Pau says I should just use shebang
instead and ignore the autoconf python stuff, since that often fails anyway.

Change-Id: Ie35dd78c42577109a6a3143221a9769e47d361a5
4 files changed