tests: Further fix bgp_l3vpn_to_bgp_vrf

tests: Further fix bgp_l3vpn_to_bgp_vrf

There still existed chances that best path consideration
has not taken place for both bgp_l3vpn_to_bgp_vrf and
bgp_instance_del_test ( since they both used the same
check_routes.py scripting ).  Add some more checks
to ensure that we have all the data.  Prior to this
change I could see one of these two tests failing
every 2-3 runs on my test system.  I am not seeing
this anymore after ~5 complete test runs.

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