content test: In prepareContentBare, don't include Scaffold by default

None of our content widgets currently depend on a Scaffold. It's not
necessarily bad if they do -- but if it ever happens, it'll be
helpful to be explicit about it, and using this param would be a way
to get that explicitness. (I assume the explicitness would be
helpful when we do zulip#488, "content: Support Zulip content outside
messages (even outside per-account contexts)".)

test/widgets/content_test.dart

@@ -92,10 +92,14 @@ void main() {
 
   TestZulipBinding.ensureInitialized();
 
-  Future<void> prepareContentBare(WidgetTester tester, String html) async {
+  Future<void> prepareContentBare(WidgetTester tester, String html, {
+    bool wrapWithScaffold = false,
+  }) async {
     Widget widget = BlockContentList(nodes: parseContent(html).nodes);
 
-    widget = Scaffold(body: widget);
+    if (wrapWithScaffold) {
+      widget = Scaffold(body: widget);
+    }
 
     await tester.pumpWidget(
       Builder(builder: (context) =>