We put a timeline of 257 to remove the old bash test runner so since
we're about to release 257, let's remove the old bash test runner in
favor of the meson + mkosi test runner.
This adds a testsuite unit to run systemd-networkd-tests.py. This is
mkosi only for now as python is not available in the images set up
by the bash framework. We give the test a lower priority as it takes
a while to run so we want to start it as soon as possible.
