Fix test_shutdown_worker leaving stale callback that breaks later tests

Root cause:
  test_shutdown_worker was an async test (@pytest.mark.asyncio), so
  pytest-asyncio ran it via loop.run_until_complete(). Inside the test,
  shutdown_worker() calls runner.close() -> loop.stop(). Calling
  loop.stop() from within loop.run_until_complete() leaves a stale
  _run_until_complete_cb in the event loop's ready queue. This callback
  calls loop.stop() when it fires, which poisons the next
  run_until_complete() call on the same loop with:
    RuntimeError: Event loop stopped before Future completed

Why the next test is affected:
  The event_loop fixture in conftest.py returns manager.get_runner().loop.
  After test_shutdown_worker, this loop still has the stale callback.
  When a later test (e.g. test_calc_job_node_get_builder_restart) calls
  run_until_complete() on the same loop, the stale callback fires
  loop.stop() prematurely.

Why the _reset_runner autouse fixture masked the issue:
  The _reset_runner fixture called manager.reset_runner() after every
  test, which called runner.close() -> loop.close(). This closed the
  loop entirely, so get_or_create_event_loop() created a fresh loop
  for the next test without the stale callback. Removing this fixture
  exposed the underlying bug.

Why run_forever fixes it:
  In the real daemon, start_daemon_worker() uses loop.run_forever(),
  not loop.run_until_complete(). run_forever() does not add a
  _run_until_complete_cb, so loop.stop() cleanly exits the loop with
  no stale callbacks. The fix changes the test to use the same pattern:
  schedule shutdown_worker as a task, then call run_forever(). This
  mirrors production behavior and avoids the stale callback.

Author: agoscinski

tests/conftest.py

@@ -55,12 +55,6 @@ class TestDbBackend(Enum):
     PSQL = 'psql'
 
 
-@pytest.fixture(autouse=True)
-def _reset_runner(request):
-    yield
-    get_manager().reset_runner()
-
-
 def pytest_collection_modifyitems(items, config):
     """Automatically generate markers for certain tests.
 

tests/engine/daemon/test_worker.py

@@ -16,17 +16,12 @@
 
 
 @pytest.mark.requires_rmq
-@pytest.mark.asyncio
-async def test_shutdown_worker(manager):
+def test_shutdown_worker(manager):
     """Test the ``shutdown_worker`` method."""
     runner = manager.get_runner()
-    await shutdown_worker(runner)
-
-    try:
-        assert runner.is_closed()
-    finally:
-        # Reset the runner of the manager, because once closed it cannot be reused by other tests.
-        manager._runner = None
+    runner.loop.create_task(shutdown_worker(runner))
+    runner.loop.run_forever()
+    assert runner.is_closed()
 
 
 @pytest.mark.usefixtures('aiida_profile_clean', 'started_daemon_client')