Arm backend: Handle empty target-less model suite

[Sebastian-Larsson]

Treat pytest status 5 as success when the target-less selector finds no tests. Preserve all other failure statuses.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils @robell @rascani

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21453

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❌ 5 New Failures

As of commit 02a5675 with merge base d0fe35d ():

NEW FAILURES - The following jobs have failed:

• pull / test-qnn-models-linux (mv2) / linux-job (gh)
  ##[error]fatal: unable to access 'https://github.com/google/re2.git/': Error in the HTTP2 framing layer
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 2b2b61070dbdea97a6ec35a6930ede7e1699f3193cdeec53aff3fc2c92cb6613 /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t fc686bf7f7cf32c747514b526dc73e909d799b2e01eb91c389bdde12cb08711c /exec failed with exit code 1
• trunk / test-models-macos-cpu (efficient_sam, portable) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / unittest-release / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 844f32d80acec97ae45239499acc4d859a188a6ba160a52eb299d0e5f5655fb8 /exec failed with exit code 1

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