Occasional test failures in `test_qt_process_events_process_all`

We are occasionally seeing this failure:
```
======================================================================
FAIL: test_qt_process_events_process_all (traitsui.testing.tests.test_gui.TestProcessEventsRepeated)
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Traceback (most recent call last):
File "D:\a\traitsui\traitsui\.edm\envs\traitsui-test-3.6-pyqt5\lib\site-packages\traitsui\testing\tests\test_gui.py", line 152, in test_qt_process_events_process_all
self.assertEqual(n_left_behind_events, 0, msg)
AssertionError: 8 != 0 : Expected 10 events processed on the objects and zero events left on the queue after running process_cascade_events. Found 2 processed with 8 left behind.

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```