Fix flakiness: finalize dropped gestures in tests to release resources, and update doc-comment. (#136136)

Reverts flutter/flutter#131998

Reverting for https://github.com/flutter/flutter/issues/132222

Fixes https://github.com/flutter/flutter/issues/59413

This relocates `mock_canvas.dart` and `recording_canvas.dart` from `flutter/test/rendering` to `flutter_test`. 

The testing functionality afforded by mock_canvas should be available to everyone, not just the framework. :)

mock_canvas.dart needed a bit of cleanup - things like formatting and super parameters.

`Checkbox.fillColor` should be applied to checkbox's background color when it is unchecked. (#125643)

* theme override fix

* add lerp condition and tests

* remove unwanted changes

* resolve app_bar token version

* update test names

* migrates lib/foundation and lib/widgets

* migrates lib/painting and lib/material

* migrates lib/cupertino, lib/gestures, lib/services, lib/semantics and lib/rendering

* fixes failing tests

* migrates test/

* improves hashing of lists

This reverts commit d056500b.

This reverts commit 31de052e.

This reverts commit e2490f29.