tests: Fix wrong interface name in SRv6 topotest

tests: Fix wrong interface name in SRv6 topotest

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

Commit 14aabc01565a918c223b7811572fae0316810422 replaced the nexthop
interface with the index of incoming VRF 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>