Updated Promise perf tests to work in CI

Great start!  With a few small tweaks, these will work in CI via a project I'm working on for automated perf testing.  

Changes:
  * Moved from `/tests/benchmark` to `/tests/performance`, which will be the location YUI tools assume performance tests are.
  * Removed the Y.Benchmark wrapper module.
  * Ditched the `assets` sub-directory.  While it is a cleaner structure for manual testing, the tool sources the JS files directly and doesn't bother with the HTML document.  For example, [scrollview perf tests](https://github.com/yui/yui3/tree/master/src/scrollview/tests/performance) don't even have HTML files.  There are of course up/down-sides to this approach, which are being evaluated.

I'll add you as a collaborator on my `yui-benchmark` repo and follow-up privately.  Would love to gather some feedback!