Add unit tests.

- [x] Add test for specifiers that shadow those on the `Object.prototype` (#78)
- [x] Add test for `.mjs` (#108)
- [ ] Add test for exporting the same identifier more than once (#106)
- [ ] Add test for passing options to `reify/node` (#95)
- [ ] Add test for `importSync` with a `key` param
- [ ] Add test for [modified ast](https://github.com/benjamn/reify/blob/a3478a7a3e8f7322f355b5358530bca959e15883/lib/import-export-visitor.js#L446-L448)
- [ ] Add test for `ast` compiler option

Start by reading the repository's existing unit-test organization, then review the linked issues #106 and #95 and the import-export visitor location referenced in the checklist. Work through the unchecked cases for duplicate exports, reify/node options, importSync with a key, modified ASTs, and the ast compiler option; done means each has a focused regression test.

Written by the indexing model from the issue text.