selftests: mptcp: join: stop transfer when check is done (part 1)

Since the "Fixes" commit mentioned below, "userspace pm" subtests of
mptcp_join selftests introduced in v6.5 are launching the whole transfer
in the background, do the required checks, then wait for the end of
transfer.

There is no need to wait longer, especially because the checks at the
end of the transfer are ignored (which is fine). This saves quite a few
seconds in slow environments.

Note that old versions will need commit bdbef0a6 ("selftests: mptcp:
add mptcp_lib_kill_wait") as well to get 'mptcp_lib_kill_wait()' helper.

Fixes: 4369c198 ("selftests: mptcp: test userspace pm out of transfer")
Cc: stable@vger.kernel.org # 6.5.x: bdbef0a6: selftests: mptcp: add mptcp_lib_kill_wait
Cc: stable@vger.kernel.org # 6.5.x
Reviewed-and-tested-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240131-upstream-net-20240131-mptcp-ci-issues-v1-8-4c1c11e571ff@kernel.org


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