mptcp: token kunit: set protocol

mptcp: token kunit: set protocol

As it would be done when initiating an MPTCP sock.

This is not strictly needed for this test, but it will be when a later
patch will check if the right protocol is being used when calling
mptcp_sk().

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240223-upstream-net-next-20240223-misc-improvements-v1-2-b6c8a10396bd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>