[ExecuTorch] Arm Ethos: Do not depend on torch.testing._internal

This can cuase issues with `disable_global_flags` and internal state of the library, this is something which is set when importing this

Differential Revision: [D70402061](https://our.internmc.facebook.com/intern/diff/D70402061/)

[ghstack-poisoned]

Author: pytorchbot