Move tests to tests/ directory

[nealrichardson]

I want to factor out the mock API responses into their own files so that they can be reused and so that they aren't taking up so much space in the test files themselves. So that we aren't cluttering the actual package submodule directories, I think it makes sense to move the tests to a tests directory.

pytest does not insist on one mode of test organization over the other: https://docs.pytest.org/en/7.1.x/explanation/goodpractices.html#choosing-a-test-layout-import-rules