Fix delete-regexp-full

By default, the `length` property of an object should not be configurable: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty#Redefining_the_length_property_of_an_Array_object

I also included an (unrelated) patch to tests/unit-tests/delete-regexp-full.js. These two patches together make the test pass.