selftests: net: Lower threshold on debug kernels for big_tcp_tunnels.sh

Debug kernels on upstream CI runners run slower and generate fewer BIG
TCP packets, making the test flaky on upstream CI runners. Lower the
default threshold for those kernels.

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/20260822120308.1165200-3-alice.kernel@fastmail.im
Signed-off-by: Jakub Kicinski <kuba@kernel.org>