tests: recover NetworkManager after kernel-mode disable_service

The kernel-mode route tests stop and restart NetworkManager per case via
disable_service. Adding a fifth case pushed the restart past systemd's
start-limit, so the unit became unactivatable and the session teardown's
veth cleanup failed with "org.freedesktop.DBus.Error.ServiceUnknown: The
name is not activatable".

reset-failed before start clears the start-limit counter, and after start
wait for NetworkManager to answer on D-Bus, since systemd reports the unit
active before it claims its bus name.

Signed-off-by: Josephine Pfeiffer <josie@redhat.com>