ipv6: add @ipv6_restart_prefixroute

This is a test for an issue, where an externally added address, that
would ordinarily cause a route to its prefix to be automatically added,
would be re-added into kernel with "noprefixroute" attribute upon
daemon restart. That would cause the automatically added route to be
removed.

The problem was fixed with NetworkManager/next merge (into 1.36).

https://bugzilla.redhat.com/show_bug.cgi?id=2010640
4 jobs for !878 with lr/prefixroute-drop in 1 minute and 2 seconds
detached
Status Name Job ID Coverage
  Test
manual TestResults #16124663
manual
failed UnitTests #16124662

00:01:02

 
  External
failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-test-upstream/2046/

failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-test-upstream/2046/

00:38:29

 
Name Stage Failure
failed
UnitTests Test
=========================== short test summary info ============================
FAILED nmci/test.py::test_feature_tags - AssertionError: tag "ipv6_restart_pr...
=================== 1 failed, 7 passed, 2 warnings in 4.06s ====================
Uploading artifacts for failed job
Uploading artifacts...
report.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=16124662 responseStatus=201 Created token=gg23bxER
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1