Add additional luhn test for fuller coverage as in #972 (#973)

* Add additional luhn test for fuller coverage as in #972

* Replace unnecessary test and add comment for future maintainers.

* Add link to issue #972 in Luhn tests

* Update luhn test formatting.

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

         assertFalse(luhnValidator.isValid("055-444-285"));
     }
 
+    /* The following test diverges from the canonical test data. This is because the corresponding canonical test does
+     * not account for Java specific functions (such as Character.getNumericValue()), which can be part of incorrect yet
+     * passing implementations. For more detail, check out issue #972 here:
+     * (https://github.com/exercism/java/issues/972).
+    */
     @Ignore("Remove to run test")
     @Test
     public void testThatStringContainingSymbolsIsInvalid() {
-        assertFalse(luhnValidator.isValid("055£ 444$ 285"));
+        assertFalse(luhnValidator.isValid("34&"));
     }
 
     @Ignore("Remove to run test")