The test expected the goroutine that picks a message
from a channel and invokes Send() to not fire fast enough
and then it would overflow.

This test, makes this deterministic by:

1) Ensuring the goroutine actually picks the message from
   the channel and is stuck on the fake gRPC stream.
2) Moves the assertion of the overflow to after sending
   a third message, so that now it overflows for sure.

Ran it 100 times and it doesn't fail now.

Change-Id: Ica9dac316f5333c0cd24dffcc1f7b1f30aa67f45
Signed-off-by: yacovm <yacovm@il.ibm.com>