selftests: mptcp: add link failure test case

selftests: mptcp: add link failure test case

Add a test case where a link fails with multiple subflows.
The expectation is that MPTCP will transmit any data that
could not be delivered via the failed link on another subflow.

Co-developed-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>