selftests: net: Add a selftest for VXLAN with FDB nexthop groups

Add test cases for VXLAN with FDB nexthop groups, testing both IPv4 and
IPv6. Test basic Tx functionality as well as some corner cases.

Example output:

 # ./test_vxlan_nh.sh
 TEST: VXLAN FDB nexthop: IPv4 basic Tx                              [ OK ]
 TEST: VXLAN FDB nexthop: IPv6 basic Tx                              [ OK ]
 TEST: VXLAN FDB nexthop: learning                                   [ OK ]
 TEST: VXLAN FDB nexthop: IPv4 proxy                                 [ OK ]
 TEST: VXLAN FDB nexthop: IPv6 proxy                                 [ OK ]

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20250901065035.159644-4-idosch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>