Missing test coverage for destructuring arguments and GetV.

[ricea]

For example getReader(null) should throw. getReader(7) should work. getReader('hi') should work, unless you set String.prototype.mode = 'not byob' first.

Similar concerns apply where the standard uses GetV.

[ricea]

If we switch to Web IDL getReader(7) will no longer work, so maybe we should hold off adding tests for it.

[domenic]

This is no longer supported after #1035, and idlharness tests it.