Fix the fetch of all example tests
# What does this PR do?

I noticed on recent PRs that when all tests are fetched, the example tests are not run. Upon closer inspection, it's because the test for `"all"` in the `test_fetcher` compares a list instead of a string. This PR addresses that.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_24864). All of your documentation changes will be reflected on that endpoint.