tests: move out of the main package.

tests: move out of the main package.

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.