BTW I don't think a separate top-level

tests

parallel source tree is officially recommended. In the Pants repo, for example, we mostly have tests right alongside the code they test (so

src/python/foo/bar.py

is tested by

src/python/foo/bar_test.py

. Pants allows you to do this because it has the information to know not to bundle tests in the distribution, and so on. We like it because it's really easy to find the tests for some module, and vice versa.