action_sheet test [nfc]: Add visibilityPolicy to a prepare

For the sake of some new callers when we move this function
definition to an outer scope.

Author: chrisbobbe

test/widgets/action_sheet_test.dart

@@ -119,6 +119,7 @@ void main() {
         String topic = someTopic,
         bool isChannelSubscribed = true,
         bool? isChannelMuted,
+        UserTopicVisibilityPolicy? visibilityPolicy,
         UnreadMessagesSnapshot? unreadMsgs,
         int? zulipFeatureLevel,
       }) async {
@@ -134,6 +135,9 @@ void main() {
           subscriptions: isChannelSubscribed
             ? [eg.subscription(effectiveChannel, isMuted: isChannelMuted ?? false)]
             : null,
+          userTopics: visibilityPolicy != null
+            ? [eg.userTopicItem(effectiveChannel, topic, visibilityPolicy)]
+            : null,
           unreadMsgs: unreadMsgs,
           zulipFeatureLevel: zulipFeatureLevel));
         store = await testBinding.globalStore.perAccount(eg.selfAccount.id);