CSHARP-4988: Fix flaky ServerMonitor test

Author: Oleksandr Poliakov

tests/MongoDB.Driver.Core.Tests/Core/Servers/ServerMonitorTests.cs

@@ -307,9 +307,10 @@ public void RoundTripTimeMonitor_should_be_started_only_once_if_using_streaming_
             var subject = CreateSubject(out var mockConnection, out _, out var mockRoundTripTimeMonitor, capturedEvents, serverMonitorSettings: serverMonitorSettings);
 
             SetupHeartbeatConnection(mockConnection, isStreamable: true, autoFillStreamingResponses: false);
-            mockConnection.EnqueueCommandResponseMessage(CreateHeartbeatCommandResponseMessage(), null);
-            mockConnection.EnqueueCommandResponseMessage(CreateHeartbeatCommandResponseMessage(), null);
-            mockConnection.EnqueueCommandResponseMessage(CreateHeartbeatCommandResponseMessage(), null);
+            // Have to setup small delay on the heartbeat responses to emulate streamableServer functionality.
+            mockConnection.EnqueueCommandResponseMessage(CreateHeartbeatCommandResponseMessage(), TimeSpan.FromMilliseconds(500));
+            mockConnection.EnqueueCommandResponseMessage(CreateHeartbeatCommandResponseMessage(), TimeSpan.FromMilliseconds(500));
+            mockConnection.EnqueueCommandResponseMessage(CreateHeartbeatCommandResponseMessage(), TimeSpan.FromMilliseconds(500));
 
             subject.Initialize();
             // spin until the initial handshake has happened in which case the server description state should be Connected