Do not re-use children task instance in test scenario

The task instance is not updated when the parent project is saved. So it keep
the former timestamp in memory which raise concurrency exceptions.

issue9101

tests/scenario_project_status.rst

@@ -56,12 +56,13 @@ Try to complete project without task::
         ...
     WorkProgressValidationError: ...
 
+    >>> task, = project.children
     >>> task.progress = 1
-    >>> task.save()
     >>> project.save()
 
 Try to re-open task without project::
 
+    >>> task = Work(task.id)
     >>> task.progress = 0.5
     >>> task.save()  # doctest: +IGNORE_EXCEPTION_DETAIL
     Traceback (most recent call last):
@@ -81,5 +82,6 @@ Change progress with updating status::
 
 Re-open task::
 
+    >>> task = Work(task.id)
     >>> task.progress = 0.5
     >>> task.save()