test: refactor test/index.js to match Node (#110)

* test: refactor test/index.js to match Node

Refactors index.js to be able to copy paste directly into the Node.js
codebase. Note: I think it is sufficient that we continue syncing just
the tests for parseArgs() to Node.js (testing the public API. This
gives us 95% coverage today, and it would be easy to push this to
100% with a couple additional tests.

Fixes #105

* Update test/index.js

Co-authored-by: Aaron Casanova <[email protected]>

Co-authored-by: Aaron Casanova <[email protected]>

Author: Benjamin E. Coe