Add tests for IteratorClose in dstr assignment (#524)

The files in this patch are organized according to the following naming
scheme:

Prefix             | Grammar production
-------------------|-------------------
`array-empty-`     | ArrayAssignmentPattern : [ ]
`array-elision-`   | ArrayAssignmentPattern : [ Elision ]
`array-rest-`      | ArrayAssignmentPattern : [ Elisionopt AssignmentRestElement ]
`array-elem-`      | ArrayAssignmentPattern : [ AssignmentElementList ]
`array-elem-trlg-` | ArrayAssignmentPattern : [ AssignmentElementList , Elisionopt AssignmentRestElementopt ]

Suffix             | Intent
-------------------|-------
`-abpt-close-err`  | The assignment evaluation returns an abrupt completion, and the iterator's `return` method throws an error
`-abpt-close-skip` | The assignment evaluation returns an abrupt completion, but IteratorClose is not invoked
`-abpt-close`      | The assignment evaluation returns an abrupt completion, and IteratorClose is invoked as specified
`-get-err`         | Abrupt completion returned from GetIterator
`-nrml-close-err`  | The assignment evaluation completes, and the iterator's `return` method throws an error
`-nrml-close-null` | The assignment evaluation completes, and the iterator's `return` method returns a non-Object value (there is no corresponding `-abpt-` suffix because the algorithm does not reference the return value in those cases)
`-nrml-close-skip` | The assignment evaluation completes, but IteratorClose is not invoked
`-nrml-close`      | The assignment evaluation completes, and IteratorClose is invoked as specified

Not all suffixes are appropriate for all productions. Suffixes have been
simplified in cases where less specificity is necessary to disambiguate
test cases.