Scan tasks should be stopped cleanly when process stopped

[GDYendell]

Acceptance Criteria

• There should be no warnings when the tests are run

Task was destroyed but it is pending!
task: <Task pending name='Task-7' coro=<_create_periodic_scan_task..scan_task() done, defined at /workspaces/FastCS/src/fastcs/backend.py:156> wait_for= cb=[_chain_future.._call_set_state() at /usr/local/lib/python3.11/asyncio/futures.py:394]>
Task was destroyed but it is pending!
task: <Task pending name='Task-8' coro=<_create_periodic_scan_task..scan_task() done, defined at /workspaces/FastCS/src/fastcs/backend.py:156> wait_for= cb=[_chain_future.._call_set_state() at /usr/local/lib/python3.11/asyncio/futures.py:394]>
Finished running tests!