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
9 jobs for !878 with lr/prefixroute-drop in 1 minute and 32 seconds
detached
Status Name Job ID Coverage
  Test
manual TestResults #16240216
manual
passed UnitTests #16240215

00:01:32

 
  External
failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-test-upstream/2058/

canceled https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-test-upstream/2070/

failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-test-upstream/2071/

failed https://jenkins-networkmanager.apps.ocp.ci.centos.org/job/NetworkManager-test-mr/847/

failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-test-upstream/2058/

00:37:30

failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-test-upstream/2071/

00:37:52

passed https://jenkins-networkmanager.apps.ocp.ci.centos.org/job/NetworkManager-test-mr/847/

00:02:51