UI Tester import side effect Test failure when run locally on wx - does not occur on CI

Running the following code generated the test error:
```
$ python etstool.py install --toolkit=wx --environment=wx-env-new
$ edm shell -e wx-env-new
$ cd traitsui/testing
$ python -m unittest
```

and the subsequent error:

```
======================================================================
ERROR: tester.qt4.tests.test_helpers (unittest.loader._FailedTest)
----------------------------------------------------------------------
ImportError: Failed to import test module: tester.qt4.tests.test_helpers
Traceback (most recent call last):
File "/Users/aayres/.edm/envs/wx-env-new/lib/python3.6/unittest/loader.py", line 428, in _find_test_path
module = self._get_module_from_name(name)
File "/Users/aayres/.edm/envs/wx-env-new/lib/python3.6/unittest/loader.py", line 369, in _get_module_from_name
__import__(name)
File "/Users/aayres/Desktop/traitsui/traitsui/testing/tester/qt4/tests/test_helpers.py", line 22, in
from traitsui.testing.tester.qt4 import _interaction_helpers
File "/Users/aayres/.edm/envs/wx-env-new/lib/python3.6/site-packages/traitsui/testing/tester/qt4/_interaction_helpers.py", line 11, in
from pyface.qt import QtCore, QtGui
File "/Users/aayres/.edm/envs/wx-env-new/lib/python3.6/site-packages/pyface/qt/__init__.py", line 83, in
raise ImportError("Cannot import PySide, PySide2, PyQt5 or PyQt4")
ImportError: Cannot import PySide, PySide2, PyQt5 or PyQt4

----------------------------------------------------------------------
```

This error does not show up on the CI.