A closure cannot be generic over the lifetime in its return type, so the
`Ctx<'_>` this one built could never be tied to the `&mut Settings` it
borrows — `lifetime may not live long enough`, and the lib test target
failed to compile.
Same shape the add_host and library test modules already use: a plain
`fn ctx<'a>(…) -> Ctx<'a>` where the lifetime is written down once.
enricobuehler