Add Skia shallow gradient test output to testing corpus.

BUG=pdfium:1152

Change-Id: I5ee5f0f3956d4999b26eaf840ac4d3dad56214a5
Reviewed-on: https://pdfium-review.googlesource.com/c/43451
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
16 files changed