flatten-array: add @Ignore to tests [Fix #379]

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

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