Add module finding support for ‘testthat’

[klmr]

The current testing workflow entails loading a submodule called __tests__ from the main module, and launching testthat::test_dir() from within it.

However, this means that although the test context sets the working directory to the tests directory, ‘box’ assumes the currently executing file is the parent module and it therefore searches for local modules in the parent directory rather than relative to the test case file.

Add support for being run inside ‘testthat’ to script_path().