Unify test cases

[reworkcss]

Unify test cases

[necolas]

Yeah, I agree. Wasn't a good idea to do it for the purposes of the merge, but it should be where we end up.

[lydell]

Do you think it is a good idea to create a subdirectory for each case in cases/, and have input.css, ast.json, identity.css and compressed.css in each subdir? The idea is to:

1. Parse input.css.
2. Make sure that the parse tree matches ast.json.
3. Stringify the parse tree using the identity compiler.
4. Make sure that the CSS output matches identity.css.
5. Stringify the parse tree using the compress compiler.
6. Make sure that the CSS output matches compress.css.

[necolas]

sgtm, was thinking the same thing.

[conradz]

Closed by #48