msglist test [nfc]: Move a Finder helper outward for reuse

Author: chrisbobbe

test/widgets/message_list_test.dart

@@ -125,6 +125,9 @@ void main() {
     return findScrollView(tester).controller;
   }
 
+  final contentInputFinder = find.byWidgetPredicate(
+    (widget) => widget is TextField && widget.controller is ComposeContentController);
+
   group('MessageListPage', () {
     testWidgets('ancestorOf finds page state from message', (tester) async {
       await setupMessageListPage(tester,
@@ -1634,9 +1637,6 @@ void main() {
     final topicNarrow = eg.topicNarrow(stream.streamId, topic);
     const content = 'outbox message content';
 
-    final contentInputFinder = find.byWidgetPredicate(
-      (widget) => widget is TextField && widget.controller is ComposeContentController);
-
     Finder outboxMessageFinder = find.widgetWithText(
       OutboxMessageWithPossibleSender, content, skipOffstage: true);