Switch tests to use wptserve

[davehunt]

As discussed in #40 we should use wptserve for tests that require a webserver.

[cuff-links]

I have gotten some experience with wptserve. Do the tests that need a web server already exist?

[davehunt]

@silne30 not yet, they'll land with the notification changes in #40. I don't think we can pick up this work until wpstserve support Python 3. There's a pull request already open for this in w3c/wptserve#93.