The py312-threading tox target will run the currently working unit tests
with threading mode. We have an exclude list, those tests are
failing or hanging. Also the current test list might still have unstable
tests.
This also adds a non voting zuul job to run the new target.
Change-Id: Ibf41fede996fbf2ebaf6ae83df8cfde35acb2b7e
Signed-off-by: Balazs Gibizer <gibi@redhat.com>