Temporarily disable sampling in `SkPngRustCodec`

This is a first step towards enabling sampling in `SkPngRustCodec`.
Right now, attempting to use sampling crashes, so temporarily disable it
entirely. Follow up CLs will first implement sampling for non-interlaced
and then interlaced images.

This change also adds some tests that will come in handy in later CLs.

BUG=378697848

Change-Id: Ied7c32991286524e2175ebfd4ccf32ae28c99175
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1279136
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Elliot Sisteron <elliotsisteron@google.com>