difference-of-squares: remove "final" from test class (#595)

* difference-of-squares: remove "final" from test class

* Update DifferenceOfSquaresCalculatorTest.java

exercises/difference-of-squares/src/test/java/DifferenceOfSquaresCalculatorTest.java

 /*
  * version: 1.1.0
  */
-public final class DifferenceOfSquaresCalculatorTest {
+public class DifferenceOfSquaresCalculatorTest {
 
     private DifferenceOfSquaresCalculator calculator;