tests: Fix verify_rib such that it will look at the selected route

tests: Fix verify_rib such that it will look at the selected route

When you have a static route with multiple different admin
distances there exists a chance that route will have been
installed multiple times due to system load when inserted
at about the same time.  If this is the case then the
verify_rib function can and will select the wrong route
that happens to have a nexthop group that is still installed.

Modify verify_rib to ensure that the route that is going to
be looked at for nexthop correctness is the actual installed
route, not a previous version of it.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>