rough unit test support for FM

I've punted for now on GPU test support.

Change-Id: I2ebe54a6fc61e1ce92120a6a221d5bb936b047e4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/255874
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed