Arm backend: Update failures to slice operator for int16x8

[SaoirseARM]

Summary

Updates the x_fail message.
Removes pytest.mark.fails from testcases and only fails on expected.

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15104

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❌ 4 New Failures, 1 Cancelled Job, 3 Unrelated Failures

As of commit 71fbef4 with merge base 57a7903 ():

NEW FAILURES - The following jobs have failed:

• Lint / android-java-format / linux-job (gh)
  RuntimeError: Command docker exec -t fe637adb46aa7ca41fa8013d226ee67486bd65c6c07fca3fbbcd5873982bd277 /exec failed with exit code 1
• Lint / lintrunner / linux-job (gh)
  >>> Lint for backends/arm/test/ops/test_slice.py:
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 77fcffc0c019e2b8c844573a63408af9ff3046d358a77e016f14b1f8cea251c9 /exec failed with exit code 1
• Test CUDA Builds / export-voxtral-cuda-artifact / linux-job (gh)
  RuntimeError: Command docker exec -t dbb34a1f000a3637d7c819a6897756170603c57236734103ff3f42f4bf72924f /exec failed with exit code 2

CANCELLED JOB - The following job was cancelled. Please retry:

• periodic (gh)

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

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

• pull / unittest / windows / windows-job (gh) (trunk failure)
  runtime/test/test_runtime_etdump_gen.py::RuntimeETDumpGenTest::test_etdump_params_with_bytes_and_buffer
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  runtime/test/test_runtime_etdump_gen.py::RuntimeETDumpGenTest::test_etdump_params_with_bytes_and_buffer
• trunk / unittest-release / windows / windows-job (gh) (trunk failure)
  runtime/test/test_runtime_etdump_gen.py::RuntimeETDumpGenTest::test_etdump_params_with_bytes_and_buffer

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[SaoirseARM]

@pytorchbot label ciflow/trunk

[SaoirseARM]

@pytorchbot label 'partner: arm'

[SaoirseARM]

@pytorchbot label "release notes: none"