possibly fixed timing issue in test

MQTTnet.Core/Server/MqttClientSessionsManager.cs

@@ -24,12 +24,10 @@ namespace MQTTnet.Core.Server
         public MqttClientSessionsManager(
             IOptions<MqttServerOptions> options, 
             ILogger<MqttClientSessionsManager> logger,
-            MqttClientRetainedMessagesManager retainedMessagesManager, 
             IMqttClientSesssionFactory clientSesssionFactory)
         {
             _logger = logger ?? throw new ArgumentNullException(nameof(logger));
             _options = options.Value ?? throw new ArgumentNullException(nameof(options));
-            RetainedMessagesManager = retainedMessagesManager ?? throw new ArgumentNullException(nameof(options));
             _clientSesssionFactory = clientSesssionFactory ?? throw new ArgumentNullException(nameof(clientSesssionFactory));
         }
 
@@ -37,8 +35,6 @@ namespace MQTTnet.Core.Server
         public event EventHandler<MqttClientDisconnectedEventArgs> ClientDisconnected;
         public event EventHandler<MqttApplicationMessageReceivedEventArgs> ApplicationMessageReceived;
 
-        public MqttClientRetainedMessagesManager RetainedMessagesManager { get; }
-
         public async Task RunClientSessionAsync(IMqttCommunicationAdapter clientAdapter, CancellationToken cancellationToken)
         {
             var clientId = string.Empty;