selftests: mptcp: launch mptcp_connect with timeout

selftests: mptcp: launch mptcp_connect with timeout

'mptcp_connect' already has a timeout for poll() but in some cases, it
is not enough.

With "timeout" tool, we will force the command to fail if it doesn't
finish on time. Thanks to that, the script will continue and display
details about the current state before marking the test as failed.
Displaying this state is very important to be able to understand the
issue. Best to have our CI reporting the issue than just "the test
hanged".

Note that in mptcp_connect.sh, we were using a long timeout to validate
the fact we cannot create a socket if a sysctl is set. We don't need
this timeout.

In diag.sh, we want to send signals to mptcp_connect instances that have
been started in the netns. But we cannot send this signal to 'timeout'
otherwise that will stop the timeout and messages telling us SIGUSR1 has
been received will be printed. Instead of trying to find the right PID
and storing them in an array, we can simply use the output of
'ip netns pids' which is all the PIDs we want to send signal to.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/160
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>