TODO: document setting 'timeout' in tests

[smikes]

When I reviewed the async test code I believe I saw a provision for overriding the default timeout value by setting the timeout attribute on a test -- either via @timeout 100 (old-style) or timeout: 100 (new-style).

Someone (me?) should verify that this exists and document it so that it can be used.

Also, some tests may be designed to time out (e.g., a test of a never-fulfilled promise), so there should be a documented way of writing an async test which is expected to time out.

All of this is necessary because we must avoid the use of setTimeout in Test262 tests.