Skip test if wx is not around

[aaronayres35]

Previously running:

python etstool.py install 
edm shell -e pyface-test-3.6-pyqt
python -m unittest

I was seeing the following test failure:

======================================================================
ERROR: pyface.ui.wx.data_view.tests.test_data_wrapper (unittest.loader._FailedTest)
----------------------------------------------------------------------
ImportError: Failed to import test module: pyface.ui.wx.data_view.tests.test_data_wrapper
Traceback (most recent call last):
  File "/Users/aayres/.edm/envs/pyface-test-3.6-pyqt/lib/python3.6/unittest/loader.py", line 428, in _find_test_path
    module = self._get_module_from_name(name)
  File "/Users/aayres/.edm/envs/pyface-test-3.6-pyqt/lib/python3.6/unittest/loader.py", line 369, in _get_module_from_name
    __import__(name)
  File "/Users/aayres/Desktop/pyface/pyface/ui/wx/data_view/tests/test_data_wrapper.py", line 21, in <module>
    @unittest.skipUnless(wx_available, "Test requires wx")
NameError: name 'unittest' is not defined

This PR simply updates that test file to skip the test if wx is not available in the environment.

Looking into this also made me realize that there is no consistent way this type of thing is done across the codebase. For example:

pyface/pyface/ui/wx/grid/tests/test_simple_grid_model.py

Lines 12 to 20 in 8cc824e

	try:
	from pyface.ui.wx.grid.api import GridRow, GridColumn, SimpleGridModel
	except ImportError:
	wx_available = False
	else:
	wx_available = True
	@unittest.skipUnless(wx_available, "Wx is not available")

pyface/pyface/action/tests/test_traitsui_widget_action.py

Line 25 in 8cc824e

@unittest.skipIf(toolkit.toolkit == "wx", "wxPython not supported")

pyface/pyface/tests/test_api.py

Lines 17 to 18 in 8cc824e

	is_wx = (ETSConfig.toolkit == 'wx')
	is_qt = ETSConfig.toolkit.startswith('qt')

pyface/pyface/tasks/tests/test_dock_pane_toggle_group.py

Line 26 in 8cc824e

USING_WX = ETSConfig.toolkit not in ["", "qt4"]

sometimes trying the import, something working with toolkit from pyface.toolkit sometimes through ETSConfig.toolkit, etc. It is pretty inconsequential (they all do what we want), but it would be good to pin down a consistent way to skip tests based on toolkit.

[rahulporuri]

It is pretty inconsequential (they all do what we want), but it would be good to pin down a consistent way to skip tests based on toolkit.

This warrants an issue - and i'm not sure which is the best option.

[rahulporuri]

LGTM