test [nfc]: Add the few missing awaits on handleEvent calls

This is NFC because there isn't currently actually anything to wait
for; the implementation of handleEvent is entirely synchronous.
But in principle it's async, because in the future it will be, for
the reasons described in baea6d7.

Thanks to Zixuan for spotting this pattern in some newly-added tests
(coming up):
  zulip#1327 (comment)

Author: gnprice

test/model/emoji_test.dart

@@ -262,14 +262,14 @@ void main() {
       ]);
     });
 
-    test('updates on RealmEmojiUpdateEvent', () {
+    test('updates on RealmEmojiUpdateEvent', () async {
       final store = prepare();
       check(store.allEmojiCandidates()).deepEquals([
         ...arePopularCandidates,
         isZulipCandidate(),
       ]);
 
-      store.handleEvent(RealmEmojiUpdateEvent(id: 1, realmEmoji: {
+      await store.handleEvent(RealmEmojiUpdateEvent(id: 1, realmEmoji: {
         '1': eg.realmEmojiItem(emojiCode: '1', emojiName: 'happy'),
       }));
       check(store.allEmojiCandidates()).deepEquals([

test/model/store_test.dart

@@ -353,26 +353,26 @@ void main() {
     group('RealmUserUpdateEvent', () {
       // TODO write more tests for handling RealmUserUpdateEvent
 
-      test('deliveryEmail', () {
+      test('deliveryEmail', () async {
         final user = eg.user(deliveryEmail: '[email protected]');
         final store = eg.store(initialSnapshot: eg.initialSnapshot(
           realmUsers: [eg.selfUser, user]));
 
         User getUser() => store.users[user.userId]!;
 
-        store.handleEvent(RealmUserUpdateEvent(id: 1, userId: user.userId,
+        await store.handleEvent(RealmUserUpdateEvent(id: 1, userId: user.userId,
           deliveryEmail: null));
         check(getUser()).deliveryEmail.equals('[email protected]');
 
-        store.handleEvent(RealmUserUpdateEvent(id: 1, userId: user.userId,
+        await store.handleEvent(RealmUserUpdateEvent(id: 1, userId: user.userId,
           deliveryEmail: const JsonNullable(null)));
         check(getUser()).deliveryEmail.isNull();
 
-        store.handleEvent(RealmUserUpdateEvent(id: 1, userId: user.userId,
+        await store.handleEvent(RealmUserUpdateEvent(id: 1, userId: user.userId,
           deliveryEmail: const JsonNullable('[email protected]')));
         check(getUser()).deliveryEmail.equals('[email protected]');
 
-        store.handleEvent(RealmUserUpdateEvent(id: 1, userId: user.userId,
+        await store.handleEvent(RealmUserUpdateEvent(id: 1, userId: user.userId,
           deliveryEmail: const JsonNullable('[email protected]')));
         check(getUser()).deliveryEmail.equals('[email protected]');
       });

test/widgets/message_list_test.dart

@@ -670,8 +670,8 @@ void main() {
         foundOldest: false, messages: messages).toJson());
     }
 
-    void handleMessageMoveEvent(List<StreamMessage> messages, String newTopic, {int? newChannelId}) {
-      store.handleEvent(eg.updateMessageEventMoveFrom(
+    Future<void> handleMessageMoveEvent(List<StreamMessage> messages, String newTopic, {int? newChannelId}) async {
+      await store.handleEvent(eg.updateMessageEventMoveFrom(
         origMessages: messages,
         newTopicStr: newTopic,
         newStreamId: newChannelId,
@@ -692,7 +692,7 @@ void main() {
         ..controller.isNotNull().text.equals('Some text');
 
       prepareGetMessageResponse([message]);
-      handleMessageMoveEvent([message], 'new topic', newChannelId: otherChannel.streamId);
+      await handleMessageMoveEvent([message], 'new topic', newChannelId: otherChannel.streamId);
       await tester.pump(const Duration(seconds: 1));
       check(tester.widget<TextField>(channelContentInputFinder))
         ..decoration.isNotNull().hintText.equals('Message #${otherChannel.name} > new topic')
@@ -722,7 +722,7 @@ void main() {
       final existingMessage = eg.streamMessage(
         stream: eg.stream(), topic: 'new topic', content: 'Existing message');
       prepareGetMessageResponse([existingMessage, message]);
-      handleMessageMoveEvent([message], 'new topic');
+      await handleMessageMoveEvent([message], 'new topic');
       await tester.pump(const Duration(seconds: 1));
 
       check(find.textContaining('Existing message').evaluate()).length.equals(1);
@@ -734,7 +734,7 @@ void main() {
       await setupMessageListPage(tester, narrow: narrow, messages: [message], streams: [channel]);
 
       prepareGetMessageResponse([message]);
-      handleMessageMoveEvent([message], 'new topic');
+      await handleMessageMoveEvent([message], 'new topic');
       await tester.pump(const Duration(seconds: 1));
 
       check(find.descendant(