tests: Fix zebra_seg6_route to not always reinstall the same route

tests: Fix zebra_seg6_route to not always reinstall the same route

This code has two issues:

a) The loop to test for successful installation re-installs
the route every time it loops.  A system under load will
have issues ensuring the route is installed and repeated
attempts does not help

b) The nexthop group installation was always failing
but never noticed (because of the previous commit)
and the test was always passing, when it should
have never passed.

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