selftests: mptcp: print_test out of verify_listener_events

verify_listener_events() helper will be exported into mptcp_lib.sh as a
public function, but print_test() is invoked in it, which is a private
function in userspace_pm.sh only. So this patch moves print_test() out of
verify_listener_events().

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240308-upstream-net-next-20240308-selftests-mptcp-unification-v1-12-4f42c347b653@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>