space-age: add hint to @Ignore annotations

ref #448

exercises/space-age/src/test/java/SpaceAgeTest.java

         assertEquals(1000000, age.getSeconds(), MAXIMUM_DELTA);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void ageOnEarth() {
         SpaceAge age = new SpaceAge(1000000000);
         assertEquals(31.69, age.onEarth(), MAXIMUM_DELTA);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void ageOnMercury() {
         SpaceAge age = new SpaceAge(2134835688);
         assertEquals(280.88, age.onMercury(), MAXIMUM_DELTA);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void ageOnVenus() {
         SpaceAge age = new SpaceAge(189839836);
         assertEquals(9.78, age.onVenus(), MAXIMUM_DELTA);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void ageOnMars() {
         SpaceAge age = new SpaceAge(2329871239L);
         assertEquals(39.25, age.onMars(), MAXIMUM_DELTA);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void ageOnJupiter() {
         SpaceAge age = new SpaceAge(901876382);
         assertEquals(2.41, age.onJupiter(), MAXIMUM_DELTA);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void ageOnSaturn() {
         SpaceAge age = new SpaceAge(3000000000L);
         assertEquals(3.23, age.onSaturn(), MAXIMUM_DELTA);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void ageOnUranus() {
         SpaceAge age = new SpaceAge(3210123456L);
         assertEquals(1.21, age.onUranus(), MAXIMUM_DELTA);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void ageOnNeptune() {
         SpaceAge age = new SpaceAge(8210123456L);