selftests: mptcp: make evts global in userspace_pm

This patch makes server_evts and client_evts global in userspace_pm.sh,
then these two variables could be used in test_announce(), test_remove()
and test_subflows(). The local variable 'evts' in these three functions
then could be dropped.

Also move local variable 'file' as a global one.
Suggested-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NGeliang Tang <geliang.tang@suse.com>
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>