Replace buggy_blend_modes GM with an exhaustive test.

The new test is disabled by default, as it's quite slow.
We can run it if we suspect problems by passing -x to DM.

This test would have been failing before the bug fix, and now is passing.

Assuming the Priv on the end means it's not considered public API...
TBR=reed@google.com

BUG=skia:4052

Review URL: https://codereview.chromium.org/1228333003
3 files changed