test: Bump recentZulipFeatureLevel to FL 334, from FL 278

In particular, this assumes support for empty topics in our app code.

To make sure we are aware of possible behavior changes in the app
code we test against.  Here's a helpful `git grep` command and its result
(thanks to "zulipFeatureLevel" being quite a greppable name):

```
$ git grep 'zulipFeatureLevel [<|>]' lib
lib/api/model/narrow.dart:  final supportsOperatorDm = zulipFeatureLevel >= 177; // TODO(server-7)
lib/api/model/narrow.dart:  final supportsOperatorWith = zulipFeatureLevel >= 271; // TODO(server-9)
lib/model/autocomplete.dart:        final isChannelWildcardAvailable = store.zulipFeatureLevel >= 247; // TODO(server-9)
lib/model/autocomplete.dart:    final isTopicWildcardAvailable = store.zulipFeatureLevel >= 224; // TODO(server-8)
lib/model/compose.dart:  final isChannelWildcardAvailable = store.zulipFeatureLevel >= 247; // TODO(server-9)
lib/model/compose.dart:  final isTopicWildcardAvailable = store.zulipFeatureLevel >= 224; // TODO(server-8)
lib/model/store.dart:    assert(zulipFeatureLevel >= 334);
lib/model/store.dart:    if (zulipFeatureLevel >= 163) { // TODO(server-7)
lib/model/store.dart:  bool get isUnsupported => zulipFeatureLevel < kMinSupportedZulipFeatureLevel;
lib/widgets/action_sheet.dart:  final supportsUnmutingTopics = store.zulipFeatureLevel >= 170;
lib/widgets/action_sheet.dart:  final supportsFollowingTopics = store.zulipFeatureLevel >= 219;
lib/widgets/action_sheet.dart:  final markAsUnreadSupported = store.zulipFeatureLevel >= 155; // TODO(server-6)
lib/widgets/actions.dart:    final useLegacy = store.zulipFeatureLevel < 155; // TODO(server-6)
lib/widgets/actions.dart:    assert(PerAccountStoreWidget.of(context).zulipFeatureLevel >= 155); // TODO(server-6)
lib/widgets/autocomplete.dart:    final isChannelWildcardAvailable = store.zulipFeatureLevel >= 247; // TODO(server-9)
lib/widgets/compose_box.dart:    if (store.zulipFeatureLevel < 334) {
lib/widgets/compose_box.dart:    if (store.zulipFeatureLevel >= 334) {
```

We can tell that this bump only affects 3 entries from above:

```
lib/model/store.dart:    assert(zulipFeatureLevel >= 334);
lib/widgets/compose_box.dart:    if (store.zulipFeatureLevel < 334) {
lib/widgets/compose_box.dart:    if (store.zulipFeatureLevel >= 334) {
```

All are related to the FL 334 (general chat) changes.

We could have bumped it further to FL 334+, but that's beyond the
needs of changes that follow.

Author: PIG208

lib/model/store.dart

@@ -577,6 +577,7 @@ class PerAccountStore extends PerAccountStoreBase with ChangeNotifier, EmojiStor
   /// be empty otherwise.
   // TODO(server-10) simplify this
   String get realmEmptyTopicDisplayName {
+    assert(zulipFeatureLevel >= 334);
     assert(_realmEmptyTopicDisplayName != null); // TODO(log)
     return _realmEmptyTopicDisplayName ?? 'general chat';
   }

test/example_data.dart

@@ -76,7 +76,7 @@ final Uri realmUrl = Uri.parse('https://chat.example/');
 Uri get _realmUrl => realmUrl;
 
 const String recentZulipVersion = '9.0';
-const int recentZulipFeatureLevel = 278;
+const int recentZulipFeatureLevel = 334;
 const int futureZulipFeatureLevel = 9999;
 const int ancientZulipFeatureLevel = kMinSupportedZulipFeatureLevel - 1;