Unmark port_forwarding tests as unstable (cont.)

Those tests were marked as unstable after migration of CI to the
Ubuntu Focal due to related bug.
Now this bug should be fixed with depends-on patch so lets make those
tests as stable again.

This is a continuation of
https://review.opendev.org/c/openstack/neutron-tempest-plugin/+/756114

Depends-On: https://review.opendev.org/756107

Change-Id: I9c592f189b0b4c69cdfcf0058502c18589f2db40
Related-Bug: #1896735
1 file changed