Tiding up the test suite

Running `make test_all` encounters a number of issues, I wanted to take the time to discuss them, and perhaps tidy up the test framework as part of that process.

As far as I can understand, the sassc test suite uses ruby sass, and the latest local built edition of sassc/libsass — then compares the output via the files `sass_output.css` & `sassc_output.css`.

`expected_output.css` is used by every test to verify any diff between rubysass and sassc. 

Now that I've outlined that — can someone confirm this? I'd like to make a `README.md` commit to describe not only how to run the test suite, but also how its implemented.