test [nfc]: Leave a few topics as explicit strings

These are being passed to test helpers which we'll leave as consuming
String instead of TopicName, for convenience in other test cases.

At these call sites the values were coming from fields that are or
will become TopicName, though; so when that happens and we make
TopicName no longer implicitly convertible to String, these will need
to be explicitly String instead.

Author: gnprice

test/widgets/compose_box_test.dart

@@ -274,7 +274,7 @@ void main() {
       final narrow = ChannelNarrow(channel.streamId);
       final destinationNarrow = eg.topicNarrow(narrow.streamId, 'test topic');
       await prepareComposeBox(tester, narrow: narrow, streams: [channel]);
-      await enterTopic(tester, narrow: narrow, topic: destinationNarrow.topic);
+      await enterTopic(tester, narrow: narrow, topic: 'test topic');
 
       await checkStartTyping(tester, destinationNarrow);
 
@@ -341,7 +341,7 @@ void main() {
       final narrow = ChannelNarrow(channel.streamId);
       final destinationNarrow = eg.topicNarrow(narrow.streamId, 'test topic');
       await prepareComposeBox(tester, narrow: narrow, streams: [channel]);
-      await enterTopic(tester, narrow: narrow, topic: destinationNarrow.topic);
+      await enterTopic(tester, narrow: narrow, topic: 'test topic');
 
       await checkStartTyping(tester, destinationNarrow);
 

test/widgets/message_list_test.dart

@@ -749,7 +749,8 @@ void main() {
   group('recipient headers', () {
     group('StreamMessageRecipientHeader', () {
       final stream = eg.stream(name: 'stream name');
-      final message = eg.streamMessage(stream: stream, topic: 'topic name');
+      const topic = 'topic name';
+      final message = eg.streamMessage(stream: stream, topic: topic);
 
       FinderResult<Element> findInMessageList(String text) {
         // Stream name shows up in [AppBar] so need to avoid matching that
@@ -808,7 +809,7 @@ void main() {
           narrow: const CombinedFeedNarrow(),
           messages: [message], subscriptions: [eg.subscription(stream)]);
         await store.handleEvent(eg.userTopicEvent(
-          stream.streamId, message.topic, UserTopicVisibilityPolicy.followed));
+          stream.streamId, topic, UserTopicVisibilityPolicy.followed));
         await tester.pump();
         check(find.descendant(
           of: find.byType(MessageList),
@@ -820,7 +821,7 @@ void main() {
           narrow: TopicNarrow.ofMessage(message),
           messages: [message], subscriptions: [eg.subscription(stream, isMuted: true)]);
         await store.handleEvent(eg.userTopicEvent(
-          stream.streamId, message.topic, UserTopicVisibilityPolicy.unmuted));
+          stream.streamId, topic, UserTopicVisibilityPolicy.unmuted));
         await tester.pump();
         check(find.descendant(
           of: find.byType(MessageList),