selftests: mptcp: improve 'fair usage on close' stability

The mentioned test has to wait for a subflow creation failure.
The current code looks for TCP sockets in TW state and sometimes
misses the relevant event. Switch to a more stable check, looking
for the associated mib counter.

Fixes: 46e967d1 ("selftests: mptcp: add tests for subflow creation failure")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/257


Reported-and-tested-by: Matthieu Baerts <matthieu.baerts@tessares.net>
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>