flatten-array: add hint to @Ignore annotations

exercises/flatten-array/src/test/java/FlattenerTest.java

                 "two")));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testASingleLevelOfNestingWithNoNulls() {
         assertEquals(
               8)));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testFiveLevelsOfNestingWithNoNulls() {
         assertEquals(
               "-2")));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testSixLevelsOfNestingWithNoNulls() {
         assertEquals(
               "8")));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testSixLevelsOfNestingWithNulls() {
         assertEquals(
               "negative two")));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testNestedListsFullOfNullsOnly() {
         assertEquals(