This is the Testo_lwt module provided by the testo-lwt library and Opam package. It adds support to Testo for test functions that return Lwt promises of type unit Lwt.t instead of returning unit. Testo_lwt's interface is otherwise identical to Testo.
Using Testo_lwt is recommended only if you use Lwt but your environment doesn't support running Lwt_run.main to turn a promise into a synchronous computation.