selftests: mptcp: join: stop transfer when check is done (part 2)

Since the "Fixes" commits mentioned below, the newly added "userspace
pm" subtests of mptcp_join selftests 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 on slow environments.

While at it, use 'mptcp_lib_kill_wait()' helper everywhere, instead of
on a specific one with 'kill_tests_wait()'.

Fixes: b2e2248f ("selftests: mptcp: userspace pm create id 0 subflow")
Fixes: e3b47e46 ("selftests: mptcp: userspace pm remove initial subflow")
Fixes: b9fb1760

 ("selftests: mptcp: userspace pm send RM_ADDR for ID 0")
Cc: stable@vger.kernel.org
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-v1-9-4c1c11e571ff@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>