tests: Honour $(NO_PYTHON) flag from Makefile in run_tests.sh

Currently the test script bases whether to run the Python tests on whether
it can see a built Python module.  That can easily be fooled if there is
a stale module there.

Instead, have it actually look at the NO_PYTHON variable exported from the
Makefile.  If the variable doesn't exist (such as if we're running the
script manually) fall back on the old logic.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>