selftests: mptcp: explicitly trigger the listener diag code-path

selftests: mptcp: explicitly trigger the listener diag code-path

The mptcp diag interface already experienced a few locking bugs
that lockdep and appropriate coverage have detected in advance.

Let's add a test-case triggering the relevant code path, to prevent
similar issues in the future.

Be careful to cope with very slow environments.

Note that we don't need an explicit timeout on the mptcp_connect
subprocess to cope with eventual bug/hang-up as the final cleanup
terminating the child processes will take care of that.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240223-upstream-net-20240223-misc-fixes-v1-10-162e87e48497@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>