selftests: net: Lower threshold with csum offload off in big_tcp_tunnels.sh

selftests: net: Lower threshold with csum offload off in big_tcp_tunnels.sh

With checksum offload disabled, much fewer BIG TCP packets are generated
due to overall loss of throughput. Use a separate threshold in these
tests, which is 1/10 of the threshold set for the rest of tests.

Fixes: 5cb53743e1ff ("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/20260822120308.1165200-4-alice.kernel@fastmail.im
Signed-off-by: Jakub Kicinski <kuba@kernel.org>