difference-of-squares: add hint to @Ignore annotation

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

         assertEquals(expected, actual);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testSquareOfSum10() {
         final int expected = 3025;
         assertEquals(expected, actual);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testSquareOfSum100() {
         final int expected = 25502500;
         assertEquals(expected, actual);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testSumOfSquares5() {
         final int expected = 55;
         assertEquals(expected, actual);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testSumOfSquares10() {
         final int expected = 385;
         assertEquals(expected, actual);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testSumOfSquares100() {
         final int expected = 338350;
         assertEquals(expected, actual);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testDifferenceOfSquares0() {
         final int expected = 0;
         assertEquals(expected, actual);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testDifferenceOfSquares5() {
         final int expected = 170;
         assertEquals(expected, actual);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testDifferenceOfSquares10() {
         final int expected = 2640;
         assertEquals(expected, actual);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testDifferenceOfSquares100() {
         final int expected = 25164150;