Restore wx CI jobs for GitHub Actions workflow

Before migration to GitHub Actions, we have CI jobs running against wxPython but they are allowed to fail (i.e. a PR still gets a green tick even if one of the wxPython jobs fail).

As of today, the following command is failing on wxPython/Ubuntu, wxPython/OSX, wxPython/Windows (See Notes) :
```
python etstool.py test
```

Currently GitHub Actions does not support allow-failure like Travis does. If we move these failing jobs to GitHub Actions, then the CI status will always show itself as a red tick, even if the wx jobs are not required to pass in order to merge a PR. With that, the "Test with EDM" excludes wxPython.

Notes: For wxPython/Windows, the test suites shipped with TraitsUI are currently passing. It is the integration tests that fail on wxPython/Windows. The job against wxPython/Windows in the "Test with PyPI" workflow does not run integration tests and _is_ passing in the weekly cron job. The integration tests are run in the "Integration tests" workflow, and that workflow currently excludes wxPython as well.

Possible actions:
(1) Remove the step to run integration tests in `python etstool.py test`, and add wxPython/Windows back to "Test with EDM". This can be done now.
(2) Add the failing CI jobs back when GitHub Actions supportsallow-failure. See https://github.com/actions/runner/issues/2347 for the popular request to support allow-failure on GitHub Actions. (Unknown timeline, as is blocked by GitHub Actions.)
(3) Fix those wx failures. (much larger impact, but also much harder)