Address another flaky test -

```
[xUnit.net 00:00:41.19]     RabbitMQ.Client.Unit.TestBlockingCell.TestBackgroundUpdateFails [FAIL]
[xUnit.net 00:00:41.19]       Assert.Throws() Failure
[xUnit.net 00:00:41.19]       Expected: typeof(System.TimeoutException)
[xUnit.net 00:00:41.19]       Actual:   (No exception was thrown)
[xUnit.net 00:00:41.19]       Stack Trace:
[xUnit.net 00:00:41.19]         D:\a\rabbitmq-dotnet-client\rabbitmq-dotnet-client\projects\Unit\TestBlockingCell.cs(170,0): at RabbitMQ.Client.Unit.TestBlockingCell.TestBackgroundUpdateFails()
  Passed RabbitMQ.Client.Unit.TestBlockingCell.TestGetValueWhichDoesTimeOut [469 ms]
  Failed RabbitMQ.Client.Unit.TestBlockingCell.TestBackgroundUpdateFails [739 ms]
  Error Message:
   Assert.Throws() Failure
Expected: typeof(System.TimeoutException)
Actual:   (No exception was thrown)
  Stack Trace:
     at RabbitMQ.Client.Unit.TestBlockingCell.TestBackgroundUpdateFails() in D:\a\rabbitmq-dotnet-client\rabbitmq-dotnet-client\projects\Unit\TestBlockingCell.cs:line 170
```

Author: lukebakken

projects/Unit/Fixtures.cs

@@ -423,6 +423,7 @@ public class TimingFixture
     {
         public static readonly TimeSpan TimingInterval = TimeSpan.FromMilliseconds(300);
         public static readonly TimeSpan TimingInterval_2X = TimeSpan.FromMilliseconds(600);
+        public static readonly TimeSpan TimingInterval_4X = TimeSpan.FromMilliseconds(1200);
         public static readonly TimeSpan SafetyMargin = TimeSpan.FromMilliseconds(150);
         public static readonly TimeSpan TestTimeout = TimeSpan.FromSeconds(5);
     }

projects/Unit/TestBlockingCell.cs

@@ -164,7 +164,7 @@ public void TestBackgroundUpdateSucceedsWithInfiniteTimeoutTimeSpan()
         public void TestBackgroundUpdateFails()
         {
             var k = new BlockingCell<int>();
-            SetAfter(TimingInterval_2X, k, 123);
+            SetAfter(TimingInterval_4X, k, 123);
 
             ResetTimer();
             Assert.Throws<TimeoutException>(() => k.WaitForValue(TimingInterval));