Create a `tests` module in `src`

[hendrikmaus]

I propose to start creating a tests module in a follow-up pr to bring the unit tests together.

See the unit tests module example directory tree in https://matklad.github.io/2021/02/27/delete-cargo-integration-tests.html

src/
  lib.rs
  tests.rs
  tests/
    foo.rs
    bar.rs

And/or see how @AlexanderThaller did the same thing in format_serde_error.

Originally posted by @hendrikmaus in #68 (comment)