Update on "[ExecuTorch] Arm Ethos: Add pass tests"

As title.

Adds pytest.test_option["tosa_ref_model"] similar to "corestone_fvp". This is a hack. Once we buckify the reference model, we should remove this. It shouldn't have impact on the OSS test coverage.

Differential Revision: [D69714010](https://our.internmc.facebook.com/intern/diff/D69714010/)

[ghstack-poisoned]

Author: digantdesai