Remove use of Thread.sleep

Using TimeUtil.<timeunit>.sleep() method is more readable and fixes the
Sonar issue.

Change-Id: I9f71aac442ace0aaed49eb1af8986147fa5c4bcc
diff --git a/src/test/java/com/ericsson/gerrit/plugins/heartbeat/HeartbeatDaemonTest.java b/src/test/java/com/ericsson/gerrit/plugins/heartbeat/HeartbeatDaemonTest.java
index 3aec912..765daac 100644
--- a/src/test/java/com/ericsson/gerrit/plugins/heartbeat/HeartbeatDaemonTest.java
+++ b/src/test/java/com/ericsson/gerrit/plugins/heartbeat/HeartbeatDaemonTest.java
@@ -29,6 +29,7 @@
 import com.google.gwtorm.client.KeyUtil;
 import com.google.gwtorm.server.SchemaFactory;
 import com.google.gwtorm.server.StandardKeyEncoder;
+import java.util.concurrent.TimeUnit;
 import org.junit.Before;
 import org.junit.Test;
 
@@ -66,7 +67,7 @@
     replay(eventDispatcherMock);
 
     heartbeatDaemon.start();
-    Thread.sleep(500);
+    TimeUnit.MILLISECONDS.sleep(500);
     verify(eventDispatcherMock);
     heartbeatDaemon.stop();
   }