Add a test for the isAvailableForEnvironment logic (#142251)

This is part 3 of a broken down version of the #140101 refactor.

There's some TODOs showing where I think we should change the behaviour, but in this PR the behaviour is unchanged.
A future PR will remove the tests that are redundant with these tests, but I wanted to make sure we had both sets in the codebase at the same time first.
This PR includes a change to the golden control test so that we can verify that these specific values do work on main. It would be extremely surprising if not, but in the interests of rigour...

packages/flutter/test/widgets/basic_test.dart

@@ -752,7 +752,7 @@ void main() {
     // golden file can be approved at any time.
     await tester.pumpWidget(RepaintBoundary(
       child: Container(
-        color: const Color(0xABCDABCD),
+        color: const Color(0x20240125),
       ),
     ));