[ExecuTorch] Arm Ethos: Add conftest TARGET

[pytorchbot]

This PR was created by the merge bot to help merge the original PR into the main branch.
ghstack PR number: #8559 by @digantdesai
^ Please use this as the source of truth for the PR details, comments, and reviews
ghstack PR base: https://github.com/pytorch/executorch/tree/gh/digantdesai/6/base
ghstack PR head: https://github.com/pytorch/executorch/tree/gh/digantdesai/6/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/main
Merge bot PR head: https://github.com/pytorch/executorch/tree/gh/digantdesai/6/orig
@diff-train-skip-merge

[pytorch-bot]

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❌ 2 New Failures

As of commit 2fb1d46 with merge base 728c255 ():

NEW FAILURES - The following jobs have failed:

• Check Labels / Check labels (gh)
  RuntimeError: Error checking labels: PR does not have required labels
• pull / test-models-linux (cmake, qwen2_5, portable, linux.2xlarge, 90) / linux-job (gh)
  RuntimeError: Model qwen2_5 is not a valid name. Available models are ['mul', 'linear', 'add', 'add_mul', 'softmax', 'dl3', 'edsr', 'emformer_transcribe', 'emformer_predict', 'emformer_join', 'llama2', 'llama', 'llama3_2_vision_encoder', 'lstm', 'mobilebert', 'mv2', 'mv2_untrained', 'mv3', 'vit', 'w2l', 'ic3', 'ic4', 'resnet18', 'resnet50', 'llava', 'efficient_sam'].

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