[REEF-1752] Increase task timeout in TestEvaluatorWithActiveContextDelayedPoison

JIRA:
  [REEF-1752](https://issues.apache.org/jira/browse/REEF-1752)

Pull Request:
  This closes #1272
diff --git a/lang/cs/Org.Apache.REEF.Tests/Functional/Failure/SleepTask.cs b/lang/cs/Org.Apache.REEF.Tests/Functional/Failure/SleepTask.cs
index 52efa78..3b6edae 100644
--- a/lang/cs/Org.Apache.REEF.Tests/Functional/Failure/SleepTask.cs
+++ b/lang/cs/Org.Apache.REEF.Tests/Functional/Failure/SleepTask.cs
@@ -40,8 +40,8 @@
 
         public byte[] Call(byte[] memento)
         {
-            Logger.Log(Level.Info, Prefix + "Will sleep for 2 seconds (expecting to be poisoned faster).");
-            Thread.Sleep(2000);
+            Logger.Log(Level.Info, Prefix + "Will sleep for 4 seconds (expecting to be poisoned faster).");
+            Thread.Sleep(4000);
             Logger.Log(Level.Info, Prefix + "Task sleep finished successfully.");
             return null;
         }