selftests: mptcp: join: more explicit check name

Before, the check names had to be very short. It is no longer the case
now that these checks are printed on a dedicated line.

Then, it looks better to have more explicit names.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240902-net-next-mptcp-mib-mpjtx-misc-v1-7-d3e0f3773b90@kernel.org


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