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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❌ 3 New Failures

As of commit a627e26 with merge base 1f114f1 ():

NEW FAILURES - The following jobs have failed:

• Build Presets / apple (macos) / build (gh)
  The process '/opt/homebrew/bin/git' failed with exit code 128
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 3307ec783c3ce83b3a65b4d37f483f3856461b10c43834e939bac6c0db2c3888 /exec failed with exit code 1
• Test CUDA Builds / export-voxtral-cuda-artifact / linux-job (gh)
  RuntimeError: Command docker exec -t f7deb05c6e816501c7e24c23723d4f5aa17991818b5f0576e95e3f71b62c464b /exec failed with exit code 2

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[SaoirseARM]

@pytorchbot label ciflow/trunk

[SaoirseARM]

@pytorchbot label 'partner: arm'

[SaoirseARM]

@pytorchbot label "release notes: none"

[Ninja91]

LGTM