selftests: net-drv: test adding flow rule to invalid RSS context

Check that adding Rx flow steering rules pointing to an RSS
context which does not exist is prevented.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250206235334.1425329-3-kuba@kernel.org


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