Add tests that force a specific CPU scan converter

This would allow us to have more test coverage on specific scan converters
which may help us detect some wierd bugs such as crbug.com/774008

Bug: chromium:774008
Change-Id: I9213b11bb47ce48a6eff8da494e1b2699d386ae9
Reviewed-on: https://skia-review.googlesource.com/60540
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
8 files changed