luhn: add hints to @Ignore annotations

exercises/luhn/src/test/java/LuhnValidatorTest.java

         assertFalse(luhnValidator.isValid("1"));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testThatTheStringConsistingOfASingleZeroIsInvalid() {
         assertFalse(luhnValidator.isValid("0"));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testThatASimpleValidNumberIsIdentifiedAsValid() {
         assertTrue(luhnValidator.isValid(" 5 9 "));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testThatAValidCanadianSocialInsuranceNumberIsIdentifiedAsValidV1() {
         assertTrue(luhnValidator.isValid("046 454 286"));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testThatAValidCanadianSocialInsuranceNumberIsIdentifiedAsValidV2() {
         assertTrue(luhnValidator.isValid("055 444 285"));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testThatAnInvalidCanadianSocialInsuranceNumberIsIdentifiedAsInvalid() {
         assertFalse(luhnValidator.isValid("046 454 287"));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testThatAnInvalidCreditCardIsIdentifiedAsInvalid() {
         assertFalse(luhnValidator.isValid("8273 1232 7352 0569"));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testThatAddingANonDigitCharacterToAValidStringInvalidatesTheString() {
         assertFalse(luhnValidator.isValid("046a 454 286"));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testThatStringContainingPunctuationIsInvalid() {
         assertFalse(luhnValidator.isValid("055-444-285"));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testThatStringContainingSymbolsIsInvalid() {
         assertFalse(luhnValidator.isValid("055£ 444$ 285"));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testThatTheStringConsistingOfASpaceAndASingleZeroIsInvalid() {
         assertFalse(luhnValidator.isValid(" 0"));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testThatStringContainingMultipleZerosIsValid() {
         assertTrue(luhnValidator.isValid(" 00000"));
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void testThatDoublingNineIsHandledCorrectly() {
         assertTrue(luhnValidator.isValid("091"));