image/gif,image/jpeg,image/png: skip FuzzDecode in testing short mode

image/gif,image/jpeg,image/png: skip FuzzDecode in testing short mode

The image/gif.FuzzDecode takes an excessive amount of time to run on various
builders - skip these in testing short mode. Likewise for image/jpeg and
image/png.

Fixes #55839

Change-Id: I1049d06b9dcbbc7dbc4f53d3c49b64e2254eabbd
Reviewed-on: https://go-review.googlesource.com/c/go/+/435175
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>