Image-based masking tests

[vaind]

Description

Currently, we test masking with standard unit tests. Because of the visual nature of the output some things are hard to verify this way, for example that a mask is rendered fully over a widget, or that the mask is not partially transparent (#2472).

I think we should add tests that save an image and compare it to a "golden" image asset saved in the test directory. This way we would be sure if anything changes (a.k.a. breaks) in masking. Adding one "golden" file before the masking is applied and another one after would help us identify flakiness (if any) that has nothing to do with masking.

See https://api.flutter.dev/flutter/flutter_test/matchesGoldenFile.html