[BUG] diffDeep doesn't handle subset tests property

**Describe the bug**

```
        assert.deepEqual(util.diffDeep([1, 2, 3], [1, 2]), { 2: undefined}); // returns {}

        let copy = {...orig};
        delete copy.elem6;

        assert.deepEqual(util.diffDeep(orig, copy), {elem6: {date: new Date, regexp: /test/i}}); // returns {}
```

This is enough bugs to change my mind from fix to deprecate, since it's unused but exposed in the API