Regroup "simple" Promise test cases.

The `promise-simple.js` tests were listed at top-level and as a result
the mocha output was a bit confusing.
Run promise-related tests in browser.

They already run in testling and node, but we'd missed adding them to
`test/index.html`.
Use latest spec for IsIterable.

See http://esdiscuss.org/topic/final-iterator-spec#content-14
and https://bugs.ecmascript.org/show_bug.cgi?id=2486