Add tests fetcher

[DN6]

What does this PR do?

Add test fetcher for PRs, similar to transformers.

Some notes:

1. LoRA related tests are omitted from the test fetcher since they have dedicated workflows. I think it would be good to always run them as well
2. Doc Tests are not included, but will be added one this PR: [tests] Doctests #5853 is merged.

Fixes # (issue)

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[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[patrickvonplaten]

Should we maybe run the test fetcher and the original tests side-by-side for a week to be sure everything works correctly?
E.g. for new we just create a new workflow, called "pr_tests_2" and merge this and then check for a week whether everything works as expected?

[DN6]

@patrickvonplaten Can do. Updating the PR.