`FlutterEngineTest.CanOverrideBackgroundColor` is flakey

I've seen `FlutterEngineTest.CanOverrideBackgroundColor` flake a fair bit the past couple of weeks.

## Example instance
See 2nd failure compared to first failure: https://ci.chromium.org/ui/p/flutter/builders/try/Mac%20Engine%20Drone/788840/overview

## related issues
- https://github.com/flutter/flutter/issues/124677

cc @cbracken 