selftests: net: Add tests for neigh_forward_grat option

Add tests to validate the neigh_forward_grat bridge option for selective
forwarding of gratuitous neighbor announcements.

The tests verify per-port and per-VLAN control of gratuitous neighbor
announcement forwarding for both IPv4 (gratuitous ARP) and IPv6
(unsolicited NA):
- When neigh_suppress is enabled with neigh_forward_grat off (default),
  gratuitous announcements are suppressed
- When neigh_forward_grat is enabled, gratuitous announcements are
  forwarded while regular neighbor discovery remains suppressed

For IPv4, use arping to send gratuitous ARP packets. For IPv6, use
mausezahn to craft unsolicited Neighbor Advertisement packets.

For the per-port tests, the IPv4 test exercises the ip link interface,
while the IPv6 test exercises the bridge link interface.
The per-VLAN tests use the bridge interface throughout, as per-VLAN
attributes are only accessible via 'bridge vlan'.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Link: https://patch.msgid.link/20260511065936.4173106-7-danieller@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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