topotests: update interface in SRv6 topotest

topotests: update interface in SRv6 topotest

Previously, routes leaked from one VRF to another VRF were associated
with the original nexthop interface.

Due to this change, the `bgp_srv6l3vpn_route_leak` topotest always fails
because it still expects the nexthop interface.

This commit fixes the expected interface name in the
`bgp_srv6l3vpn_route_leak` topotest.

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>