matrix: add hint to @Ignore annotations

exercises/matrix/src/test/java/MatrixTest.java

         }
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @RunWith(Parameterized.class)
     public static class CountColumnsTest {
         private Matrix matrix;
         }
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @RunWith(Parameterized.class)
     public static class GetFirstRowTest {
         private Matrix matrix;
         }
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @RunWith(Parameterized.class)
     public static class GetLastRowTest {
         private Matrix matrix;
         }
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @RunWith(Parameterized.class)
     public static class GetFirstColumnTest {
         private Matrix matrix;
     }
 
 
-    @Ignore
+    @Ignore("Remove to run test")
     @RunWith(Parameterized.class)
     public static class GetLastColumnTest {
         private Matrix matrix;