Fix unit tests

The unit tests rely on a version of the `DomUtils` module that is newer
than the one currently packaged with `HtmlParser2` [1]. Include
`DomUtils` directly so the tests may run to completion.

[1]
https://github.com/jugglinmike/node-htmlparser/blob/2c568d36d4af04de7754e3194429c238f3793acf/package.json#L25