allow skipped source/backend pairs in fm

GMs sometimes want to skip instead of drawing or failing.
E.g. GPU-only GMs with a non-GPU backend.

Change-Id: I5bd8acecbef1aa88b4d7e6cfb88dfde04424aa8b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/205349
Reviewed-by: Mike Klein <mtklein@google.com>
1 file changed