chore: remove deprecated `extern` declaration and flatten test module

[andrewbanchich]

Description of changes:

• removed extern crate since it's no longer needed: https://doc.rust-lang.org/edition-guide/rust-2018/path-changes.html
• flattened module structure of test

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.