Avoid run tests on branch `gh-pages` in Heroku

The `gh-pages` is managed by `MkDocs`, I not want every time `MkDocs` is deployed with `Github Actions` with the command `mkdocs gh-deploy --force`, because that branch never ever would contains tests, that cause the CI in `Heroku` fail each time this branch is updated, but that is a bad approach and ends by waste the resources and money of `Heroku` and me