[ExecuTorch] Arm Ethos: Add conftest TARGET (#8673)

Pull Request resolved: https://github.com/pytorch/executorch/pull/8559

As title.
ghstack-source-id: 268178172
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Differential Revision: [D69714021](https://our.internmc.facebook.com/intern/diff/D69714021/)

Co-authored-by: Digant Desai <digantdesai@meta.com>