added test for script & when the parent is created in a mixin

On Sass 3.3 alpha 255, Sass seems to lose certain parts of the parent selector when it's interpolated in Sass script: that is, in a nested mixin, it seems to lose any rule nodes that were created in containing mixins. More info and sample code in #930. Sorry I can't help any further -- I did clone a copy of the repo onto my computer and tried to fish through the code, but I think that fixing this issue would demand a lot more knowledge of the code than I have!