test: Enable hit-test failures as errors, not warnings

Author: gnprice

test/flutter_test_config.dart

@@ -0,0 +1,12 @@
+import 'dart:async';
+
+import 'package:flutter_test/flutter_test.dart';
+
+Future<void> testExecutable(FutureOr<void> Function() testMain) async {
+  // Enable these checks.  See this property's doc, which is basically
+  // an apology for the fact that (for historical reasons) it isn't
+  // the default.
+  WidgetController.hitTestWarningShouldBeFatal = true;
+
+  await testMain();
+}