bgpd, tests: fix route leaking from the default l3vrf

bgpd, tests: fix route leaking from the default l3vrf

Leaked route from the l3VRF are installed with the loopback as the
nexthop interface instead of the real interface.

> B>* 10.0.0.0/30 [20/0] is directly connected, lo (vrf default), weight 1, 00:21:01

Routing of packet from a L3VRF to the default L3VRF destined to a leak
prefix fails because of the default routing rules on Linux.

> 0:      from all lookup local
> 1000:   from all lookup [l3mdev-table]
> 32766:  from all lookup main
> 32767:  from all lookup default

When the packet is received in the loopback interface, the local rules
are checked without match, then the l3mdev-table says to route to the
loopback. A routing loop occurs (TTL is decreasing).

> 12:26:27.928748 ens37 In  IP (tos 0x0, ttl 64, id 26402, offset 0, flags [DF], proto ICMP (1), length 84)
>     10.0.0.2 > 10.0.1.2: ICMP echo request, id 47463, seq 1, length 64
> 12:26:27.928784 red   Out IP (tos 0x0, ttl 63, id 26402, offset 0, flags [DF], proto ICMP (1), length 84)
>     10.0.0.2 > 10.0.1.2: ICMP echo request, id 47463, seq 1, length 64
> 12:26:27.928797 ens38 Out IP (tos 0x0, ttl 63, id 26402, offset 0, flags [DF], proto ICMP (1), length 84)
>     10.0.0.2 > 10.0.1.2: ICMP echo request, id 47463, seq 1, length 64

Do not set the lo interface as a nexthop interface. Keep the real
interface where possible.

Fixes: db7cf73a33 ("bgpd: fix interface on leaks from redistribute connected")
Fixes: 067fbab4e4 ("bgpd: fix interface on leaks from network statement")
Fixes: 8a02d9fe1e ("bgpd: Set nh ifindex to VRF's interface, not the real")
Fixes: https://github.com/FRRouting/frr/issues/15909
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>