add flatten test to SkFont

Change-Id: I7e96e826fad9aa204c963ae671cce668a676378f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253684
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed