Also add more test cases.

Change-Id: I53cc6484b25560fc7a4b5d44e73bbd9270c25769
Reviewed-on: https://go-review.googlesource.com/59950Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>