For #1313, adds import-index-into-index test

This is for https://github.com/LearnBoost/stylus/issues/1313. I get `Failed to locate @import file index.styl` when I try for `index.styl` but `Maximum call stack size exceeded` when I try for `a.styl`. When I actually run the tests on OS X, they pass, though. Unfortunately I couldn’t get the tests to run on Windows.