selftests: mptcp: make evts global in mptcp_join

This patch moves evts_ns1 and evts_ns2 out of do_transfer() as two global
variables in mptcp_join.sh. Init them in init() and remove them in
cleanup().

Add a new helper reset_with_events() to save the outputs of 'pm_nl_ctl
events' command in them. And a new helper kill_events_pids() to kill
pids of 'pm_nl_ctl events' command. Use these helpers in userspace pm
tests.

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>