Stop using external test packages in Workhorse

This MR removes the last uses of external test packages (package foo_test) in Workhorse.

External test packages can be useful to exercise the public interface of a library or to break import cycles. Workhorse is not a library so there is no use for the first use case. In practice we have no use for the second use case either.

Besides having genuine uses, external test packages are also preferred by some people as a matter of style. However the overall test style in Workhorse is to not use external test packages (unless necessary) so for the sake of consistency, we should not have unnecessary uses of external test packages.

In the past we have been less consistent in matters of style so some older packages did use external test packages. This MR refactors those packages to use internal tests instead, making them fit in the with overall code style of Workhorse.