Support for nested test file structures

[telegott]

Thanks for this promising package!

It seems that there is more and more momentum behind moving away from the traditional package structure that is fundamentally limited with regards to best practices and is not the most sensible choice in all situations.

A hindrance for working fully "outside" the package structure is the lack of support from some tools, e.g. testthat. I checked rhino's code for testing, but the testthat::test_dir function only test files in one folder, not in nested ones, which would mean that the tests/testthat folder could not mirror the file tree in R, which I think would be a standard way to structure things.

Are there plans to work around this limitation, e.g. combining the results of multiple test_dir runs?