topotests: fix invalidate exported vpn prefixes on srv6l3vpn vrf3 setup

topotests: fix invalidate exported vpn prefixes on srv6l3vpn vrf3 setup

When srv6 is disabled due to misconfiguration, exported VPN prefixes
are invalidated, except for the ones that have their nexthop modified
with the 'nexthop vpn export' command. The previous commit also
invalidates those vpn prefixes.

Apply the changes to the test by not considering some prefixes as
selected. Enforce the expected route count.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>