selftests/net: fix out-of-order delivery of FIN in gro:tcp test

selftests/net: fix out-of-order delivery of FIN in gro:tcp test

Due to the gro_sender sending data packets and FIN packets
in very quick succession, these are received almost simultaneously
by the gro_receiver. FIN packets are sometimes processed before the
data packets leading to intermittent (~1/100) test failures.

This change adds a delay of 100ms before sending FIN packets
in gro:tcp test to avoid the out-of-order delivery. The same
mitigation already exists for the gro:ip test.

Fixes: 7d1575014a63 ("selftests/net: GRO coalesce test")
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Anubhav Singh <anubhavsinggh@google.com>
Link: https://patch.msgid.link/20251030062818.1562228-1-anubhavsinggh@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>