Ensure gc fixture is used in custreamz test (#18915)

This fixes the issue @mroeschke spotted in #18887 (comment), where this fixture wasn't actually being used.

Authors:
  - Tom Augspurger (https://github.com/TomAugspurger)

Approvers:
  - Matthew Roeschke (https://github.com/mroeschke)

URL: #18915

Author: TomAugspurger

python/custreamz/custreamz/tests/test_dataframes.py

@@ -35,6 +35,9 @@ def disable_distributed_gc_diagnosis():
     distributed.gc.enable_gc_diagnosis()
 
 
+pytestmark = pytest.mark.usefixtures("disable_distributed_gc_diagnosis")
+
+
 @pytest.fixture(scope="module")
 def client():
     client = Client(processes=False, asynchronous=False)