Arm backend: Add --dump_artifacts pytest option

[tom-arm]

• Add --dump_artifacts option to dump all artifacts of all selected tests
• --dump_artifacts option accepts path to write artifacts to

Change-Id: Ia08bce8b93a4dc178fc87c58234788bf41906205

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell @rascani

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21842

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❌ 1 New Failure, 1 Unrelated Failure

As of commit 48b11de with merge base 79e3eae ():

NEW FAILURE - The following job has failed:

• trunk / test-multimodal-macos (gemma3-4b) / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Apple / build-demo-ios / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 65

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