tests: Fix test_bgp_vpnv6_per_nexthop_label.py to handle timing changes

tests: Fix test_bgp_vpnv6_per_nexthop_label.py to handle timing changes

So the test script is making changes to a vpn configuration by
changing something fundamental about the vpn.  This is causing
a window where routes we are interested in are:
present ( from pre-change ) then
withdrawn ( the test change causes this ) then
present ( with the new data )

The test code was trying to test for this by checking
to see if the prefix was there, but due to timing issues
it's not always there when we look for it.

Modify the test to get the vpn table version prior to
the change( as that it should not be moving around ) and
then change the test for the prefix to look for a version
that is later than the vpn's table version.  Then we know
that it is *after* everything has stabilized again.

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