selftests: mptcp: add fastclose testcase

This patch added the self test for MP_FASTCLOSE. Reused the argument
addr_nr_ns2 of do_transfer() to pass the extra arguments '-I 2' to
mptcp_connect commands. Then mptcp_connect disconnected the
connections to trigger the MP_FASTCLOSE sending and receiving. Used
chk_fclose_nr to check the MP_FASTCLOSE mibs and used chk_rst_nr to
check the MP_RST mibs. This test used the test_linkfail value to make
1024KB test files.

The output looks like this:

Created /tmp/tmp.XB8sfv1hJ0 (size 1024 KB) containing data sent by client
Created /tmp/tmp.RtTDbzqrXI (size 1024 KB) containing data sent by server
001 fastclose test                syn[ ok ] - synack[ ok ] - ack[ ok ]
                                  ctx[ ok ] - fclzrx[ ok ]
                                  rtx[ ok ] - rstrx [ ok ]   invert

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>