selftests: mptcp: join: mute errors when ran in the background

The test is supposed to be killed before the end, which will likely
cause "Connection reset by peer" errors. It is confusing, especially
because in case of real transfer errors, the test will not be marked as
failed. But that's OK, there are many other tests checking that.

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-v1-9-d3e0f3773b90@kernel.org


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