Add utility class tests (#871)

Checks for classes that only have static methods.

wpilibj/src/main/java/edu/wpi/first/wpilibj/RobotController.java

@@ -18,6 +18,7 @@ import edu.wpi.first.wpilibj.hal.PowerJNI;
  */
 public final class RobotController {
   private RobotController() {
+    throw new UnsupportedOperationException("This is a utility class!");
   }
 
   /**