The test needed to be fixed because of how the completer behaviour
changed.
Before:
completer always scheduled a completion update on selection changed,
but the updates themselves were ignored if not needed.
Now:
completer only schedules completion updates when actually needed, but
never ignores a completion update.
So, before it was correct to check whether `set_model` was called, now
we must check if the completion was actually scheduled. This can be
done by checking the parameters with which `_change_completed_part`
is called, since a completion is scheduled only when `immediate=True`
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