TestReadStdin always fill up buffer provided by ReadFile caller full.
But we do not know if real ReadFile does the same. Add tests where
buffer is only filled with limited data.

Change-Id: I0fc776325c2b1fe60511126c439f4b0560e9d653
Reviewed-on: https://go-review.googlesource.com/33030Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>