selftests: mptcp: userspace: avoid read errors

During the cleanup phase, the server pids were killed with a SIGTERM
directly, not using a SIGUSR1 first to quit safely. As a result, this
test was often ending with two error messages:

  read: Connection reset by peer

While at it, use a for-loop to terminate all the PIDs the same way.

Also the different files are now removed after having killed the PIDs
using them. It makes more sense to do that in this order.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>