selftests: mptcp: listener test for in-kernel PM

This patch adds test coverage for listening sockets created by the
in-kernel path manager in mptcp_join.sh.

It adds the listener event checking in the existing "remove single
address with port" test. The output looks like this:

 003 remove single address with port syn[ ok ] - synack[ ok ] - ack[ ok ]
                                     add[ ok ] - echo  [ ok ] - pt [ ok ]
                                     syn[ ok ] - synack[ ok ] - ack[ ok ]
                                     syn[ ok ] - ack   [ ok ]
                                     rm [ ok ] - rmsf  [ ok ]   invert
                                     CREATE_LISTENER 10.0.2.1:10100[ ok ]
                                     CLOSE_LISTENER 10.0.2.1:10100 [ 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>