[Bug] Fix flaky tests

[dandavison]

The test suite currently has multiple tests that sometimes fail. Let's collect examples in this ticket, and fix them.

[dandavison]

https://github.com/temporalio/sdk-python/actions/runs/10403635533/job/28817496590?pr=609

FAILED tests/worker/test_activity.py::test_sync_activity_process_non_picklable_heartbeat_details - assert "Can't pickle" in "AttributeError: Can't get local object 'picklable_activity_non_pickable_heartbeat_details.<locals>.<lambda>'"

[dandavison]

https://github.com/temporalio/sdk-python/actions/runs/12930726515/job/36062930291?pr=739

poe test -s --workflow-environment time-skipping --junit-xml=junit-xml/3.13--macos-intel--time-skipping.xml

FAILED tests/worker/test_workflow.py::test_unfinished_handler_on_workflow_termination[-cancellation---no-wait-all-handlers-finish---not-dynamic---late-registered---signal-] - Failed: Timeout >600.0s

[dandavison]

https://github.com/temporalio/sdk-python/actions/runs/12939859391/job/36092964653?pr=739

=========== 342 passed, 10 skipped, 9 warnings in 316.04s (0:05:16) ============
FATAL: exception not rethrown
Error: Process completed with exit code 250.

[dandavison]

FAILED tests/worker/test_workflow.py::test_workflow_patch_memoized - Failed: Timeout >600.0s

https://github.com/temporalio/sdk-python/actions/runs/9745494374/job/26893565606?pr=566
https://github.com/temporalio/sdk-python/actions/runs/9745832979/attempts/1?pr=567

[cretz]

(accidentally reopened)