Add comment on why the test ServerWithJMXTest is disabled (#6338)

* Add comment on why this test is disabled
* Update demos/embedded/src/test/java/org/eclipse/jetty/demos/ServerWithJMXTest.java

Signed-off-by: olivier lamy <oliver.lamy@gmail.com>
Co-authored-by: Jan Bartel <janb@webtide.com>

demos/embedded/src/test/java/org/eclipse/jetty/demos/ServerWithJMXTest.java

@@ -27,7 +27,7 @@ import org.junit.jupiter.api.Test;
 
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
-@Disabled
+@Disabled(value = "ServerWithJMX uses registry on fixed port 1999 which can be already in-use")
 public class ServerWithJMXTest extends AbstractEmbeddedTest
 {
     private Server server;