Add tests

[melissawm]

For now, the documentation itself serves as tests for the current features, but this doesn't test (for example) the use of symlinked directories. We should have a proper test suite for the extension.

[JWCook]

I think there are two options for this:

1. Unit test: mock everything (Sphinx app and config objects, etc.), and test individual classes and functions
2. Integration test: Use a full build environment with some test source files and expected HTML output

Option 1 would probably be more straightforward to write (without much knowledge of Sphinx internals), but option 2 would be more thorough. I'm not sure how best to set up a build environment for testing, but I'm curious, so I'm willing to look into it.

Sphinx has a sphinx.testing subpackage that doesn't appear to be documented, but looks potentially useful: https://github.com/sphinx-doc/sphinx/tree/master/sphinx/testing

[JWCook]

sphinx.testing looks like the best option for most of the things we would want to test. If that sounds good to you, I'll plan on getting that started with a few basic tests.

[melissawm]

I'd be happy to see that, thanks!