In the "reusable workflow expansion" case (that is, when `runs-on` is not provided), this PR adds tests for `on.workflow_call.outputs`, and expands the test case to have two layers of reusable workflows to cover quirky behaviours that were discovered during manual testing.
Manual invocation successful:

Reviewed-on: https://code.forgejo.org/forgejo/end-to-end/pulls/1322
Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Co-committed-by: Mathieu Fenniak <mathieu@fenniak.net>