pyface/ui/wx/data_view/tests/test_data_wrapper.py tests should be skipped if wx not installed

[aaronayres35]

the tests in this file: https://github.com/enthought/pyface/blob/master/pyface/ui/wx/data_view/tests/test_data_wrapper.py should all be skipped if wx is not installed to avoid the tests failing when running the test suite with a different backend.

This is related to #812. We would want to do something like what is done here: https://github.com/enthought/pyface/blob/92ab7363268ddcf9efde8fc4cdc561b042160e90/pyface/ui/wx/grid/tests/test_simple_grid_model.py#L12-L20

[kitchoi]

I think there are probably many tests like this. On CI, etstool.py sets an environment variable: https://github.com/enthought/pyface/blob/92ab7363268ddcf9efde8fc4cdc561b042160e90/etstool.py#L294-L299 which gets used in load_tests for filtering test suites during unittest discover: https://github.com/enthought/pyface/blob/92ab7363268ddcf9efde8fc4cdc561b042160e90/pyface/init.py#L69 e.g. the test_data_wrapper has a path python.ui.wx.data_view.tests.test_data_wrapper, which would match the (wx) pattern and be skipped.

For me, I don't use etstool.py 99% of the time, so I really dislike the environment variable approach: It means the test suite does not work nicely out of the box unless one knows of this hidden switch. However, see also https://github.com/enthought/pyface/issues/662#issuecomment-671452413

[aaronayres35]

I think there are probably many tests like this.

I just tested with wx and there are many occurrences of failures with ImportError: Cannot import PySide, PySide2, PyQt5 or PyQt4

[kitchoi]

In this PR that introduces GitHub Actions to TraitsUI, the environment variable is made redundant: https://github.com/enthought/traitsui/pull/1415

I just noticed that the recipes for building EDM eggs also have to set this environment for their tests. So the environment variable setup also affects distribution packagers.