test: fix app/socket sporadic failure

The patch fixes the following test failure:

 | --- app/socket.result	Mon Feb 25 17:32:49 2019
 | +++ app/socket.reject	Mon Feb 25 17:39:51 2019
 | @@ -2827,7 +2827,7 @@
 |  ...
 |  echo_fiber ~= nil
 |  ---
 | -- true
 | +- false
 |  ...
 |  client:write('hello')
 |  ---

This happens, because we don't wait for echo_fiber to start.
Use a channel to make sure it does. Also, increase read/write
timeouts from 0.1 up to 5 seconds - it won't increase the test
runtime, but it will make it more robust.

Closes #4022