etl: add hints to @Ignore annotations

exercises/etl/src/test/java/EtlTest.java

         assertEquals(expected, etl.transform(old));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testTransformMoreValues() {
         Map<Integer, List<String>> old = new HashMap<Integer, List<String>>() {
         assertEquals(expected, etl.transform(old));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testMoreKeys() {
         Map<Integer, List<String>> old = new HashMap<Integer, List<String>>() {
         assertEquals(expected, etl.transform(old));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testFullDataset() {
         Map<Integer, List<String>> old = new HashMap<Integer, List<String>>() {