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

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.

Updated unit tests for `Checkbox` to have M2 and M3 versions.

More info in #127064

Fixes https://github.com/flutter/flutter/issues/117627

### Behavior changes:
1. If fade in/fade out animation is already in progress, hiding/showing the tooltip will immediately take effect without waiting for `waitDuration`.
2. A PointerDownEvent that doesn't become a part of a "trigger" gesture dismisses the tooltip, even for hovered ones.
3. The OverlayEntry is now updated only when the previous tooltip was completely dismissed. This can be fixed by OverlayPortal but I'm not sure what the correct behavior is.

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

Re-open from https://github.com/flutter/flutter/pull/116551

This PR added optional labelText (which will be  rendered side by side with Checkbox in the future, and thus is also announced by default by screen readers), and semanticLabel(which will be announced by screen reader if provided, overrides labelText, in order to do that we might want to wrap the Text widget inside ExcludeSemantics in the future).

Issues fixed:
[b/239564167]

Add adaptive Checkbox and CheckboxListTile

* Fix typos

* lowercase animated & opacity

* Undo typo fix

---------
Co-authored-by: Michael Goderbauer <goderbauer@google.com>

* remove newline

* update crswap

* add const constructor

* update const

* update m3 values

* update test formatting

* update crswap

* update test

* update token value

* update tests

* Add error state support for side property

* lint fixes

* lint fixes

* remove down position from toggleable ripple

* Introduce tests for tristate checkboxes

* Initial

* Use tristate changes in engine

* Flutter_test matchers test update

* Comments, tests

* Update packages/flutter/lib/src/semantics/semantics.dart
Co-authored-by: Chris Bracken <chris@bracken.jp>

* Assert mutual exclusivity

* Assert valid state before updating state

* Update packages/flutter/lib/src/semantics/semantics.dart

Typo fix in comment
Co-authored-by: Loïc Sharma <737941+loic-sharma@users.noreply.github.com>
Co-authored-by: Chris Bracken <chris@bracken.jp>
Co-authored-by: Loïc Sharma <737941+loic-sharma@users.noreply.github.com>

This reverts commit 5fd259be.

This reverts commit d056500b.

This reverts commit 31de052e.

This reverts commit e2490f29.