test_focus failure on macOS

`test_focus` consistently fails for me on macOS when run as part of the whole test suite using etstool. Following a `python etstool.py install` and a `python etstool.py test`, I get this output:
```
======================================================================
FAIL: test_focus (pyface.tests.test_widget.TestConcreteWidget)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/mdickinson/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface/tests/test_widget.py", line 333, in test_focus
self.assertFalse(self.widget.has_focus())
AssertionError: True is not false

----------------------------------------------------------------------
Ran 1196 tests in 56.754s

FAILED (failures=1, skipped=60)
```

This test is apparently already skipped on Linux and Windows. I couldn't find any issue associated with that skip; I think if there isn't one, we should open one (or repurpose this one) and add that issue reference to the skip message.