selftests: drv-net: test the napi init state

Test that threaded state (in the persistent NAPI config) gets updated
even when NAPI with given ID is not allocated at the time.

This test is validating commit ccba9f6b ("net: update NAPI threaded
config even for disabled NAPIs").

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Joe Damato <joe@dama.to>
Link: https://patch.msgid.link/20250815013314.2237512-1-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>