Add local diff output for SkiaGoldPixelDiff

This CL adds support for the gtest-based skiagold tests to automatically
call `goldctl diff` and print out the locations of the resulting
generated, closest, and diff images. This will only happen locally on
test failure and use the `--skia-gold-local-png-write-directory` flag
as the base directory.

Bug: 1444923
Change-Id: I0e9915743ea6cdfdc08cb26f1119d1e412c82212
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4540800
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Michael Tang <tangm@microsoft.com>
Reviewed-by: Sven Zheng <svenzheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1168346}