action_sheet test: Cut assumption of topic recipient header in topic narrow

With zulip#1039 "Fuse recipient header into app bar for topic narrows",
this setup will likely become invalid. It assumes that a topic
recipient header can be found in a topic-narrow message list.

Switching to the combined-feed narrow would break tests that use a
muted topic, because the topic's message(s) are excluded in that
narrow, so there wouldn't be a recipient header to tap.

Instead, trigger the topic action sheet from the topic-narrow app
bar.

Author: chrisbobbe

test/widgets/action_sheet_test.dart

@@ -277,17 +277,7 @@ void main() {
 
         final message = eg.streamMessage(
           stream: someChannel, topic: topic, sender: eg.otherUser);
-        connection.prepare(json: eg.newestGetMessagesResult(
-          foundOldest: true, messages: [message]).toJson());
-        await tester.pumpWidget(TestZulipApp(accountId: eg.selfAccount.id,
-          child: MessageListPage(
-            initNarrow: eg.topicNarrow(someChannel.streamId, topic))));
-        await tester.pumpAndSettle();
-
-        await tester.longPress(find.descendant(
-          of: find.byType(RecipientHeader), matching: find.text(topic)));
-        // sheet appears onscreen; default duration of bottom-sheet enter animation
-        await tester.pump(const Duration(milliseconds: 250));
+        await showFromAppBar(tester, channel: someChannel, topic: topic, message: message);
       }
 
       void checkButtons(List<Finder> expectedButtonFinders) {