selftests: net: add support for testing SO_RCVMARK and SO_RCVPRIORITY

Introduce tests to verify the correct functionality of the SO_RCVMARK and
SO_RCVPRIORITY socket options.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Suggested-by: Ferenc Fejes <fejes@inf.elte.hu>
Signed-off-by: Anna Emese Nyiri <annaemesenyiri@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250214205828.48503-1-annaemesenyiri@gmail.com


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