image/jpeg: prepare for new FDCT/IDCT implementations

image/jpeg: prepare for new FDCT/IDCT implementations

Preparation for a new unencumbered implementation of fdct.go and idct.go.

- Change benchmark not to allocate O(b.N) storage.
- Make tests point out where differences are.
- Parameterize differ tolerance.

Change-Id: Id5dfee40f86de894bad72dd6178ba61ec81ea2da
Reviewed-on: https://go-review.googlesource.com/c/go/+/705516
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>