Currently, /task_manager/task_status_recursive/{task_id} and
/task_manager/task_status/{task_id} unregister queries task if it
has already finished.
The status should not disappear after being queried. Do not unregister
finished task when its status or recursive status is queried.
(cherry picked from commit 18cc79176a)
After c1b2b8cb2c /task_manager/wait_task/
does not unregister tasks anymore.
Delete the check if the task was unregistered from test_task_manager_wait.
Check task status in drain_module_tasks to ensure that the task
is removed from task manager.
Fixes: #19351.
Closesscylladb/scylladb#19834
Task manager api will be used in many tests. Thus, to make it easier
api calls to task manager are wrapped into functions in task_manager_utils.py.
Some helpers that may be reused in other tests are moved there too.