selftests: mptcp: validate MPJoinRejected counter

selftests: mptcp: validate MPJoinRejected counter

The parent commit adds this new counter, incremented when receiving a
connection request, if the PM didn't allow the creation of new subflows.

Most of the time, it is then kept at 0, except when the PM limits cause
the receiver side to reject new MPJoin connections. This is the case in
the following tests:

 - single subflow, limited by server
 - multiple subflows, limited by server
 - subflows limited by server w cookies
 - userspace pm type rejects join
 - userspace pm type prevents mp_prio

Simply set join_syn_rej=1 when checking the MPJoin counters for these
tests.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250413-net-next-mptcp-sched-mib-sft-misc-v2-6-0f83a4350150@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>