More thorough tests

[dpvc]

When making the changes for PR #284, I had to change the test file for it, and that got me thinking about the tests for mathjax-node. Currently, the tests like this one only check that a particular field is present in the results, but a more thorough test would be to check that exactly the expected fields are present. So if you asked for mmlNode, one would not just check for results.mmlNode, but also that no other values were present in results. What do you think?

I suppose what we really need is a comprehensive test suite that tests the actual features and their expected results. All we have now are tests for specific issues and their fixes.

[pkra]

On a related note, stratifying test titles and filenames is something I've been itching to do.