When I review this from top to bottom, at this point I think there's only a single test case that verifies that `Likeness<Concretetype>.ToResemblance()` doesn't return null.

However, a simple way to pass this particular test case would be to return null for `Likeness<TypeWithPrivateDefaultCtorAndOtherCtor<string>>.ToResemblance()`.

Why not expand this test case and verify that the actual result is an actual instance of `TypeWithPrivateDefaultCtorAndOtherCtor<string>`?