Fix #413 Stop tests failing randomly due to issues with reading buffers too quickly

Author: w0rp

test/test_command_chain.vader

@@ -28,10 +28,12 @@ Before:
   \   {
   \     'callback': 'RunFirstEcho',
   \     'output_stream': 'stdout',
+  \     'read_buffer': 0,
   \   },
   \   {
   \     'callback': 'RunSecondEcho',
   \     'output_stream': 'stdout',
+  \     'read_buffer': 0,
   \   },
   \ ],
   \})