Since test files don't exceed 10KiB, print the full context of the diff,
including bytes that are equal.
Also, fix the labels for got and want; they were backwards before.

Change-Id: Ibac022e5f988d26812c3f75b643cae8b95603fc9
Reviewed-on: https://go-review.googlesource.com/55151Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>