msglist test [nfc]: Pin down lack of compose box in no-insets regression test

This way if some future change causes CombinedFeedNarrow to have a
compose box, we'll be prompted to update this test, so that it still
checks for this bug if there are any other narrows that still lack
a compose box.

Author: gnprice

test/widgets/message_list_test.dart

@@ -122,15 +122,20 @@ void main() {
   });
 
   group('presents message content appropriately', () {
-    // regression test for https://github.com/zulip/zulip-flutter/issues/736
-    testWidgets('content in "Combined feed" not asked to consume insets (including bottom)', (tester) async {
+    testWidgets('content not asked to consume insets (including bottom), even without compose box', (tester) async {
+      // Regression test for: https://github.com/zulip/zulip-flutter/issues/736
       const fakePadding = FakeViewPadding(left: 10, top: 10, right: 10, bottom: 10);
       tester.view.viewInsets = fakePadding;
       tester.view.padding = fakePadding;
 
       await setupMessageListPage(tester, narrow: const CombinedFeedNarrow(),
         messages: [eg.streamMessage(content: ContentExample.codeBlockPlain.html)]);
 
+      // Verify this message list lacks a compose box.
+      // (The original bug wouldn't reproduce with a compose box present.)
+      final state = MessageListPage.ancestorOf(tester.element(find.text("verb\natim")));
+      check(state.composeBoxController).isNull();
+
       final element = tester.element(find.byType(CodeBlock));
       final padding = MediaQuery.of(element).padding;
       check(padding).equals(EdgeInsets.zero);