Use "loaded" instance of Cheerio in unit tests

This is not going to make for fun review work, but since @davidchambers [asked for it](https://github.com/cheeriojs/cheerio/pull/502#discussion_r13399298), I nominate him! This is mostly a trivial transformation, but it did require a couple substantive changes. I'll call those out with in-line comments.

Commit message:

> This usage allows tests to be slightly more concise. In addition, it
> better reflects the real-world usage of Cheerio, promoting the unit
> tests' role as active usage examples.