linked list: add hint to @Ignore annotations

Ref. #448

exercises/linked-list/src/test/java/DoublyLinkedListTest.java

         assertThat(subject.pop(), is(10));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testPushShift() {
         subject.push(10);
         assertThat(subject.shift(), is(20));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testUnshiftShift() {
         subject.unshift(10);
         assertThat(subject.shift(), is(10));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testUnshiftPop() {
         subject.unshift(10);
         assertThat(subject.pop(), is(20));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testExample() {
         subject.push(10);