selftests: mptcp: add the MP_PRIO testcases

This patch added the MP_PRIO testcases:

Add a new argument bkup for run_tests and do_transfer, it can be set as
"backup" or "nobackup", the default value is "".

Add a new function chk_prio_nr to check the MP_PRIO related MIB counters.

The output looks like this:

29 single subflow, backup      syn[ ok ] - synack[ ok ] - ack[ ok ]
                               ptx[ ok ] - prx   [ ok ]
30 single address, backup      syn[ ok ] - synack[ ok ] - ack[ ok ]
                               add[ ok ] - echo  [ ok ]
                               ptx[ ok ] - prx   [ 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>