Fix a bug in AwaitNextNotificationAsync that prevented the 'Waited fo… (

#5386)

### Changes

Fix a bug in AwaitNextNotificationAsync that prevented the 'Waited for'
log line to show up, to help debug
#5384

### Testing

Since this was a bug in the testing code, I'm OK with not testing it.

<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

Source/DafnyLanguageServer.Test/Util/TestNotificationReceiver.cs

@@ -52,10 +52,10 @@ public class TestNotificationReceiver<TNotification> {
  }
  }
 
- public Task<TNotification> AwaitNextNotificationAsync(CancellationToken cancellationToken) {
+ public async Task<TNotification> AwaitNextNotificationAsync(CancellationToken cancellationToken) {
  var start = DateTime.Now;
  try {
- return notifications.Dequeue(cancellationToken);
+ return await notifications.Dequeue(cancellationToken);
  } catch (OperationCanceledException) {
  var last = History.Any() ? History[^1].Stringify() : "none";
  logger.LogInformation($"Waited for {(DateTime.Now - start).Seconds} seconds for new notification.\n" +