process_cascade_events occasionally fails to process all on some Qt versions and platforms

The following failure occurs a few times on the Appveyor build e.g. [#932](https://github.com/enthought/traitsui/pull/932) but it looks obviously unrelated:
```
======================================================================
FAIL: test_qt_process_events_process_all (traitsui.tests.test__tools.TestProcessEventsRepeated)
----------------------------------------------------------------------
Traceback (most recent call last):
File "C:\Users\appveyor\.edm\envs\traitsui-test-3.6-pyside2\lib\site-packages\traitsui\tests\test__tools.py", line 113, in test_qt_process_events_process_all
self.assertEqual(q_object.n_events, max_n_events)
AssertionError: 2 != 10
```

It is not reproducible on an actual Windows machine.