python: use avocado's "new" runner

The old legacy runner no longer seems to work with output logging, so we
can't see failure logs when a test case fails. The new runner doesn't
(seem to) support Coverage.py yet, but seeing error output is a more
important feature.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Beraldo Leal <bleal@redhat.com>
Message-id: 20220119193916.4138217-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/avocado.cfg

 [run]
-test_runner = runner
+test_runner = nrunner
 
 [simpletests]
 # Don't show stdout/stderr in the test *summary*