tests: Fix race condition in send overflow test

This change ensures that the compositor process is not able to respond
to any of the noop requests sent by the client process, by using the
test compositor's `stop_display` mechanism to coordinate when the
compositor should stop processing messages.

(Before this change, it was possible that one of the calls of
wl_event_loop_dispatch in the compositor process could respond to all
the client's noop requests before returning.)

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>