selftests: net: amt: wait longer for connection before sending packets

selftests: net: amt: wait longer for connection before sending packets

Both send_mcast4() and send_mcast6() use sleep 2 to wait for the tunnel
connection between the gateway and the relay, and for the listener
socket to be created in the LISTENER namespace.

However, tests sometimes fail because packets are sent before the
connection is fully established.

Increase the waiting time to make the tests more reliable, and use
wait_local_port_listen() to explicitly wait for the listener socket.

Fixes: c08e8baea78e ("selftests: add amt interface selftest script")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Link: https://patch.msgid.link/20260120133930.863845-1-ap420073@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>