[ Widget Previews ] Add widget_preview_scaffold.shard to test the widget_preview_scaffold template contents (#166358)

Adds a new `widget_preview_scaffold.shard` directory which contains a
hydrated `widget_preview_scaffold` template. This will allow for us to
write widget tests against the widgets defined in the templates.

This PR doesn't add any widget tests and is only adding the ability to
run these tests in follow up changes.

Fixes https://github.com/flutter/flutter/issues/166416

dev/bots/test.dart

@@ -236,10 +236,17 @@ Future<void> _runIntegrationToolTests() async {
   );
 }
 
+Future<void> _runWidgetPreviewScaffoldToolTests() async {
+  await runFlutterTest(
+    path.join(_toolsPath, 'test', 'widget_preview_scaffold.shard', 'widget_preview_scaffold'),
+  );
+}
+
 Future<void> _runToolTests() async {
   await selectSubshard(<String, ShardRunner>{
     'general': _runGeneralToolTests,
     'commands': _runCommandsToolTests,
+    'widget_preview_scaffold': _runWidgetPreviewScaffoldToolTests,
   });
 }