Make ProcessorOptimizationTest for canTweakAlphaForCoverage more robust

Change-Id: Ic43bebee6a0036eff5b718cc505d85bf839cb8a3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/260898
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
1 file changed