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

[kitchoi]

The following failure occurs a few times on the Appveyor build e.g. #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.

[kitchoi]

It is weird that we are seeing more of this lately, we got occurrences from:
#955
#957

[kitchoi]

One with PyQt5, so it is not just PySide2.

[kitchoi]

This is the first time I have seen a different error message where the actual number of events wasn't 2, but this was with PyQt4 and a Travis CI build on OSX:

======================================================================
FAIL: test_qt_process_events_process_all (traitsui.tests.test__tools.TestProcessEventsRepeated)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/travis/.edm/envs/traitsui-test-3.6-pyqt/lib/python3.6/site-packages/traitsui/tests/test__tools.py", line 110, in test_qt_process_events_process_all
    self.assertEqual(q_object.n_events, 1)
AssertionError: 3 != 1

(Note that this is on #971, and "pyqt" means PyQt4)

Edited: This error assertion here is a sanity check. process_cascade_events hasn't been called yet.

[kitchoi]

Turns out this test failure is very reproducible with PyQt4 and Mac OSX! (Discovered via #971)