Fix lazy variables loading in blocks.

This branches fixed the lazy loading of variables inside a block.

It also adds updateam tests for lazy loading.

Do we need to add the minified test for each less file?
I was thinking that a separate / single test for minifcation should be enough.

It also adds the foundation for testing token and parsing for variables.

Some tests are failing since I had a WTF moment. Sometimes the value for a variable is a single string, but othe times is a tuple as css_ident.

I can remove those test to have this branch landed, but I would like to discuss how you would like to handle varialbles declaration.

Thanks.

Let me know if something needs to be changes.