More font paths test coverage.

Specifically, the new cases check the correctness of paths generated by font_files / font_url when relative_assets is on in build mode, and when a custom fonts_dir is specified in preview mode.

I wrote these cases while investigating a bug where absolute font paths are generated despite relative_assets being set. The bug doesn't seem to be here (I'll look at middleman-sprockets next), but the tests seem useful to have anyway.