topotests: fix bgp_vpnv[4,6]_per_nexthop prefix not updated

topotests: fix bgp_vpnv[4,6]_per_nexthop prefix not updated

The bgp_vpnv[4,6]_table_check() functions analyze the
expected label value of VPN prefixes present in the BGP table.
However, it doesn't verify if the prefixes exist before doing
this. Consequently, the tests will fail if the prefixes do not
show up immediately.
Ensure that all expected VPN prefixes are present before
executing the function.

Fixes: ae5a6bc1f6ba ("topotests: add bgp mpls allocation per next-hop test")
Fixes: 37a02a8dcb0d ("topotests: add bgp_vpnv6 test allocation")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>