selftests: mptcp: add backup with port testcase

This patch added the backup testcase using an address with a port number.

The original backup tests only work for the output of 'pm_nl_ctl dump'
without the port number. It chooses the last item in the dump to parse
the address in it, and in this case, the address is showed at the end
of the item.

But it doesn't work for the dump with the port number, in this case, the
port number is showed at the end of the item, not the address.

So implemented a more flexible approach to get the address and the port
number from the dump to fit for the port number case.

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