Add React version test setup validation test

test/unit/react_version_test.dart

@@ -0,0 +1,27 @@
+@JS()
+library react_version_test;
+
+import 'package:js/js.dart';
+import 'package:test/test.dart';
+
+void main() {
+  // This test helps us verify our test setup, ensuring
+  // we're running on the React version we expect to be.
+  test('Setup check: window.React is React 18', () {
+    expect(reactVersion, startsWith('18.'));
+  });
+
+  // TODO: Once we do a dual-CI setup, we can conditionall verify this using tags
+  // test('Setup check: window.React is React 17', () {
+  //   expect(reactVersion, startsWith('17.'));
+  // }, tags: 'react-17');
+  // test('Setup check: window.React is React 18', () {
+  //   expect(reactVersion, startsWith('18.'));
+  // }, tags: 'react-18');
+  // Then, when running tests:
+  // - on React 17: `dart run build_runner test -- ... --exclude-tags=react-18`
+  // - on React 18: `dart run build_runner test -- ... --exclude-tags=react-17`
+}
+
+@JS('React.version')
+external String get reactVersion;