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
Prevent transient failures caused by slow test execution.

Testling would occasionally fail the `Promise.race` test because test
execution on the browser under test was so slow that all the delayed
promises would complete "at once".  Ensure that even if this happens
the test gives the correct answer.
Fix IE9 failures in String#startsWith and String#endsWith.

Skip the tests which require strict mode when running on IE9.

Closes #232.