internal: add test case for multiple JS files

* ensures that the transform is called on each file
* ensures the default class names contain folder names

Author: joeybaker

tests/cases/multiple-js-files/expected.css

@@ -0,0 +1,3 @@
+._simple_styles__foo {
+  color: #F00;
+}

tests/cases/multiple-js-files/main.js

@@ -0,0 +1 @@
+module.exports = require('../simple/main.js');