selftests: mptcp: stop tests earlier

These 'endpoint' tests from 'mptcp_join.sh' selftest start a transfer in
the background and check the status during this transfer.

Once the expected events have been recorded, there is no reason to wait
for the data transfer to finish. It can be stopped earlier to reduce the
execution time by more than half.

For these tests, the exchanged data were not verified. Errors, if any,
were ignored but that's fine, plenty of other tests are looking at that.
It is then OK to mute stderr now that we are sure errors will be printed
(and still ignored) because the transfer is stopped before the end.

Fixes: e274f715

 ("selftests: mptcp: add subflow limits test-cases")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>