More depth tracking

`SbyTask::update_status()` optionally takes the current step/depth, which gets used for some solvers/engines when exiting to pass unknown properties.
btor engine tracks the current step, but it doesn't track/report which property triggered a CEX so it's only useful on exit.
Use data source as a fallback if engine isn't provided (such as when coming from the `task_status` instead of an engine update).

sbysrc/sby_status.py

@@ -243,7 +243,7 @@ class SbyStatusDb:
                 round(now - self.start_time, 2),
                 self.task.name,
                 self.task.opt_mode,
-                data.get("engine", "ENGINE?"),
+                data.get("engine", data["source"]),
                 property.hdlname,
                 property.location,
                 property.status,