Arm backend: Increased pytest timeout

[Christoffer-JL]

Increased the pytest timeout 1200s->1800s
due to timeout on tiny stable diffusion

Change-Id: I64e618a2eb60ff038538bdd6567de18152ff8c1b

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

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18631

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❌ 1 New Failure, 2 Pending, 2 Unrelated Failures

As of commit 92c4568 with merge base 37ea3ff ():

NEW FAILURE - The following job has failed:

• trunk / test-huggingface-transformers-macos (smollm2-135m|coreml_fp32_gpu|--quantize) / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-models-linux (emformer_transcribe, portable, linux.2xlarge) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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

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

• trunk / unittest-release / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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