Async invoke tasks inherit the running-engine context, so raise_ from them is treated as internal #707

Why

Tasks spawned for an async invoke copy the _running_engine ContextVar from the step that spawned them. A raise_() from inside such a task is then taken as an internal event of a macrostep that already ended, instead of going to the external queue.

Scope

Reproduce with a test on the async engine; run invoke tasks outside the running-engine context.

Refs #692