Unmodified expected_failing examples are not covered by md5 check

... and thus get executed for each build, which seems improvable.

This behavior has been mentioned in https://github.com/smarie/mkdocs-gallery/issues/34, with a detailed analysis in [this comment](https://github.com/smarie/mkdocs-gallery/pull/39#issuecomment-1079731480).

The [PR fixing the issue](https://github.com/smarie/mkdocs-gallery/pull/39) has been merged recently.