selftests: mptcp: listener test for userspace PM

This patch adds test coverage for listening sockets created by userspace
processes.

It adds a new test named test_listener() and a new verifying helper
verify_listener_events(). The new output looks like this:

 CREATE_SUBFLOW 10.0.2.2 (ns2) => 10.0.2.1 (ns1)              [OK]
 DESTROY_SUBFLOW 10.0.2.2 (ns2) => 10.0.2.1 (ns1)             [OK]
 MP_PRIO TX                                                   [OK]
 MP_PRIO RX                                                   [OK]
 CREATE_LISTENER 10.0.2.2:37106				      [OK]
 CLOSE_LISTENER 10.0.2.2:37106				      [OK]

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>