NM - ipv6 - fix 'keep route upon reapply' to teardown properly
tests bugs:
- https://bugzilla.redhat.com/show_bug.cgi?id=2004212
- https://bugzilla.redhat.com/show_bug.cgi?id=2006408
- https://bugzilla.redhat.com/show_bug.cgi?id=2007264
The new tests are essetially Beniamino Galvani's reproducer (https://bugzilla.redhat.com/show_bug.cgi?id=2004212#c3 ) in nmci-ish, first tests just route presence after reapply, second test also tests address presence after timeout.