python: enable direct invocation of pytest
python: enable direct invocation of pytest

Add the pythonpath option to our pytest configuration so that it can
successfully find our package in src/cockpit.

This allows us to directly type `pytest` to test the files as they
appear in the source directory.  That's enough for many purposes, but
won't catch issues caused by broken packaging.

Indeed: we specifically want this *not* to happen when we use tox — in
that case, only the copy installed into the virutalenv should be found.
Neutralise the option with the `-opythonpath=` command-line option.

While we're at it, add the test path.  The automatic detection was
currently doing a reasonable job, but only because the unit tests in
test/verify/ didn't have .py suffix.  Let's make it more explicit.

cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit