Place tests alongside their code (#969)

Reorg of the tests: move them just alongside their code. The principle relates to grouping by coupling, not by function and is established in best-practice documents (e.g. https://github.com/focusaurus/express_code_structure#underlying-principles-and-motivations), despite its break from the tradition of a separate `test/` tree. All of today's tools can handle tests spread through the repository.

There are some good, if subtle consequences of this change:

- Since files are close at hand, friction is reduced at development time, which encourages that new tests are written to cover new behaviors.
- It's easier to find the tests that cover a particular piece of functionality.
- It's easier to see which code has tests and which doesn't.