Compared to tests in the repository root, tests in the package have
many downsides:
* Unless explicitly excluded in find_packages(), tests and their
support code effectively become a part of public API.
This, unfortunately, happened with FHDLTestCase, which was never
intended for downstream use.
* Even if explicitly excluded from the setuptools package, using
an editable install, or setting PYTHONPATH still allows accessing
the tests.
* Having a sub-package that is present in the source tree but not
exported (or, worse, exported only sometimes) is confusing.
* The name `nmigen.test` cannot be used for anything else, such as
testing utilities that *are* intended for downstream use.
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