selftests: mptcp: add testcase for active-back

selftests: mptcp: add testcase for active-back

Add more test-case for link failures scenario,
including recovery from link failure using only
backup subflows and bi-directional transfer.

Additionally explicitly check for stale count

Co-developed-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>