Update Leap tests to match canonical data in v1.4.0 (#516)

New test added in
exercism/problem-specifications#1359

Author: PakkuDon

exercises/leap/leap.spec.js

@@ -16,4 +16,8 @@ describe('A leap year', () => {
   xtest('year divisible by 400: leap year', () => {
     expect(isLeap(2000)).toBeTruthy();
   });
+
+  xtest('year divisible by 200, not divisible by 400: common year', () => {
+    expect(isLeap(1800)).toBeFalsy();
+  });
 });