selftests: mptcp: add testcases for ADD_ADDR with port

This patch adds testcases for ADD_ADDR with port and the related MIB
counters check in chk_add_nr. The output looks like this:

 24 signal 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 ]
 25 subflow and signal with port       syn[ ok ] - synack[ ok ] - ack[ ok ]
                                       add[ ok ] - echo  [ ok ] - pt [ ok ]
                                       syn[ ok ] - synack[ ok ] - ack[ ok ]
                                       syn[ ok ] - ack   [ ok ]
 26 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 ] - sf    [ ok ]

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>