Convert css! plugin test to intern.

Redesigned test to:
- not depend on css.js internals such as stylesheet insertion order
- be resilient to being run from different URL's (browser vs. command line)
- be resilient to race conditions due to other widgets
  loading, and thus loading their own CSS, while this test is running.

Also removed IE special path code in css.js; appears not to be necessary, even in IE9.