Register PNG decoder in some tests

In http://review.skia.org/1000976 and https://crrev.com/c/6607867
the Skia Fontations code now needs a PNG decoder to be explicitly
registered. Some tests were not registering it, and so this fixes
those. Registering codecs is not thread safe, so it's guarded in
a std::once.

Change-Id: I93749a4af272404b6c14c122535c770467905c09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6617367
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: Kaylee Lubick <kjlubick@chromium.org>
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Kaylee Lubick <kjlubick@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1481841}