Tiny improvements to the test suite

Hello,

This is just a tiny pull request that ships with tiny improvements in the test suite:

* Define a `Redcarpet::TestCase` class to gather any helper or any specific snippets.
* Move the `html_equal` helper to the latter.
* A nitpick for the `HTML_TOC` render's test &mdash; let's rely on `#scan` instead of `#split` as the number of expected items is more clear.

Let me know what do you think.

Have a nice day.