Create golden test to verify the rendering output of the fix for text with matrix gradients

The tests included in [the indicated PR](https://github.com/flutter/engine/pull/33131) are all API tests ("did it create the object we intended it to create?").

There should be a golden test that makes sure the output matches what we expected.