[rust png] Reland: Reinforce `testonly`-ness of some users of `libpng`.

This reverts commit 7fa4c2d7ab42a48247ce8e0290cbb7e854990f1e and
effectively relands https://crrev.com/c/6976870.  The reland is
attempted after using wildcards to cover extra gni-generated targets.
`Cq-Include-Trybots` directives in the CL description should verify
correctness of the reland.  Original CL description follows:

This CL extracts a separate `third_party/libpng:libpng_for_testonly`
target which:

1) Is a `group` that proxies/forwards to the `libpng` target by listing
   it as its `public_deps`
2) Has limited visibility (based on the new `libpng_testonly_visibility`
   introduced in the refactored `third_party/libpng/visibility.gni`).
   The new visibility is almost unchanged, except for explicitly
   spelling out some targets instead of using wildcards
3) Is marked as `testonly`

Bug: 443128323
Change-Id: If7ba0d184324a5c662aaf2d52122994ea778d452
Cq-Include-Trybots: luci.chromium.try:ios-catalyst
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6991491
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1522830}
NOKEYCHECK=True
GitOrigin-RevId: 4f7637304eca894adf2e70078a55654a88224a30