[Backend Tester] Write report progressively

[GregoryComer]

Append to the report file line by line after each test, rather than all at the end. This ensures that report data is available if the test run is aborted or hit with an unrecoverable native crash.

[GregoryComer]

Stack from ghstack (oldest at bottom):

• [Backend Tester] Seed based on test name #13313
• [Backend Tester] Reduce log verbosity / spam #13312
• [Backend Tester] Add subtest index field #13311
• Link Vulkan backend with pybinding lib when built #13309
• -> [Backend Tester] Write report progressively #13308
• [Backend Tester] Clean up report output #13306
• [Backend Tester] Clean up a few test issues #13258
• [Backend Tester] Add portable test flow #13250
• [Backend Tester] Report PTE size #13249
• [Backend Tester] Report delegation statistics #12846
• [Backend Tester] Report quantization and lowering times #12838
• [Backend Tester] Add tensor error statistic reporting #12809
• [Backend Tester] Add CSV report generation #12741
• [Backend Tester] Add Qualcomm tester and register flow #12739

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13308

• 📄 Preview Python docs built from this PR

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❌ 3 New Failures

As of commit 54563ee with merge base 3a02146 ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold
• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh)
  backends/arm/test/models/stable_diffusion/test_vae_AutoencoderKL.py::TestAutoencoderKL::test_AutoencoderKL_tosa_MI
• pull / unittest-arm-backend-with-no-fvp (test_pytest_ops) / linux-job (gh)
  RuntimeError: Command docker exec -t ef1820776d0d0fd0705a5cf2cacaf7c7fc443800bb66ead5def53b44509452f1 /exec failed with exit code 1

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