* 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>