Add tests ensuring iterator is not closed (#702)

A subtle aspect of the for-of iteration protocol concerns abrupt
completions that do *not* trigger iterator closing. Although this detail
is implicit in the current structure of the specification text, some
hosts may violate the protocol by closing the iterator because later
steps *do* specify that behavior.

The V8 engine is one such host--as of this writing, it incorrectly
closes the iterator when accessing the `value` property of the iterator
result produces an abrupt completion.

Add tests verifying that the iterator protocol is not violated in this
way for abrupt completions during the semantics of for-of evaluation.