wx test interactons? / test_key_click_backspace_with_selection failure when running full test suite

to reproduce:

```
python etstool.py install --toolkit=wx --editable
edm shell -e traitsui-test-3.6-wx
python -m unittest
```

many tests fail (😞 ), but specifically I see:
```
======================================================================
FAIL: test_key_click_backspace_with_selection (traitsui.testing.tester._ui_tester_registry.wx.tests.test_interaction_helpers.TestInteractions)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/aayres/Desktop/traitsui/traitsui/testing/tester/_ui_tester_registry/wx/tests/test_interaction_helpers.py", line 162, in test_key_click_backspace_with_selection
self.assertEqual(textbox.Value, "E")
AssertionError: 'ABCD' != 'E'
- ABCD
+ E
```

However, if I run:
`python -m unittest traitsui/testing/tester/_ui_tester_registry/wx/tests/test_interaction_helpers.py -v`
all tests pass, specifically
`test_key_click_backspace_with_selection (traitsui.testing.tester._ui_tester_registry.wx.tests.test_interaction_helpers.TestInteractions) ... ok`

This suggests this test is somehow being affected by other tests in the test suite?