Test decoding and output failures to JSON in get_images_from_skps.

Testing for decoding was copy pasted from msarett's CL https://codereview.chromium.org/1828323002/

BUG=skia:4981
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1844713003

Review URL: https://codereview.chromium.org/1844713003
2 files changed