Function check_engine_found 

pub fn check_engine_found(
    kind: EngineKind,
    detect: &dyn Fn(EngineKind) -> Result<Engine, Unavailable>,
) -> (EngineCheck, Option<Engine>)

The same question as check_engine, with the engine that answered handed back beside the answer and detect put in place of the machine.

The engine is kept rather than looked up again, for the reason Gates::probe_with gives. An engine that cannot be used has none to hand back, so the answer there is Broken with nothing beside it.

Blocks on starting the engine, so it belongs on a background thread.