selftests: net: add test for passive TFO socket NAPI ID

Add a test that checks that the NAPI ID of a passive TFO socket is valid
i.e. not zero.

Signed-off-by: David Wei <dw@davidwei.uk>
Link: https://patch.msgid.link/20250617212102.175711-4-dw@davidwei.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>