selftests: net: Wait for netserver to launch

Use wait_local_port_listen after starting netserver in
big_tcp_tunnels.sh to ensure it's listening when the test starts.

Fixes: 5cb53743e1 ("selftests: net: Add a test for BIG TCP in UDP tunnels")
Signed-off-by: Alice Mikityanska <alice@isovalent.com>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260822120308.1165200-2-alice.kernel@fastmail.im
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/big_tcp_tunnels.sh

@@ -3,6 +3,8 @@
 #
 # Testing for IPv4 and IPv6 BIG TCP over VXLAN and GENEVE tunnels.
 
+source "$(dirname "$0")/lib.sh"
+
 SERVER_NS=$(mktemp -u server-XXXXXXXX)
 SERVER_IP4="192.168.1.1"
 SERVER_IP6="2001:db8::1:1"
@@ -39,6 +41,7 @@ setup() {
 		gro_max_size 196608 gro_ipv4_max_size 196608
 
 	ip netns exec "$SERVER_NS" netserver >/dev/null
+	wait_local_port_listen "$SERVER_NS" 12865 tcp
 }
 
 setup_tunnel() {