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.
This obscure functionality was likely only ever used in old MiSoC
code, and doesn't justify the added complexity. It was also not
provided (and could not be reasonably provided) in SyncFIFOBuffered,
which made its utility extremely marginal.
test_signed and test_coding are adjusted slightly to account for
differences in comb propagation between the simulators; we might want
to revert that eventually.
