Support for other test folder structures

In many frameworks, rails for example, the tests or specs are placed in a separate folder from the implementation. The folder structure of the tests often mirrors that of the implementation. Would like this project to support that.

The issue names no files, tests, or entry points. Start by locating how the project currently maps implementation files to tests, then inspect existing test-discovery coverage; done means supporting separate, mirrored test or spec folders without breaking current layouts.

Written by the indexing model from the issue text.