This improves the short version of the writer test.

First of all, it has a much quicker setup. Previously that
could take up towards 0.5 second.

Secondly, it will test all compression levels in short mode as well.

Execution time is 1.7s/0.03s for normal/short mode.

Change-Id: I275a21f712daff6f7125cc6a493415e86439cb19
Reviewed-on: https://go-review.googlesource.com/21800Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>