selftests: drv-net: psp: add basic data transfer and key rotation tests

Add basic tests for sending data over PSP and making sure that key
rotation toggles the MSB of the spi.

Deploy PSP responder on the remote end. We also need a healthy dose
of common helpers for setting up the connections, assertions and
interrogating socket state on the Python side.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Link: https://patch.msgid.link/20250927225420.1443468-5-kuba@kernel.org


Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>