Skip Wx test failures / dont allow CI to fail

Currently, the test suite is allowed to fail on wx, but this adds the risk of us introducing regressions without realizing it (merging green PRs that actually introduce new wx failures).

We want to halt this now, skipping any currently failing tests on wx (they can be fixed over time), but requiring the remaining test suite to pass. This way we will catch new failures before the causing code gets merged.

See: https://github.com/enthought/pyface/pull/900#pullrequestreview-608287222