[Backend Test] Backend test reporting skeleton

[GregoryComer]

Summary

Add the initial skeleton of reporting code for the backend tester. This PR is primarily focused on putting the hooks and runner structure in place. Follow-up work will expand the scope of collection and reporting outputs.

This PR adds the following:

• CLI runner for the test suite.
• Basic test result breakdown by success / fail and cause (failing in lowering vs output mismatch, for example).
• Refactoring of test suite logic to clean things up.

Next steps:

• Aggregate results by flow (backend).
• Add additional CLI flags to allow filtering backends and dtypes.
• Land more of the operator test suite.
• Wire up flows for quantized operators.

Note that this PR is stacked on (and thus includes) #11960. I accidently broke my ghstack, so I'm converting this to a normal PR.

Sample output (XNNPACK):

Test Session Summary:

   84 Passed / 95
   11 Failed / 95
    0 Skipped / 95

[Success]
   66 Delegated
   18 Undelegated

[Failure]
    4 Lowering Fail
    0 PTE Load Fail
    0 PTE Run Fail
    6 Output Mismatch Fail

Reproduce with ET_TEST_ENABLED_BACKENDS=xnnpack python -m executorch.backends.test.suite.runner.executorch.backends.test.suite. I've temporarily commented out non-f32 dtypes to work around some crashes in XNNPACK, which are non-recoverable from Python.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12296

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure, 2 Unrelated Failures

As of commit 05f933d with merge base f9a3ca8 ():

NEW FAILURE - The following job has failed:

• Lint / lintrunner / linux-job (gh)
  >>> Lint for backends/test/suite/runner.py:

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

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

• pull / test-models-linux (mobilebert, portable, linux.2xlarge) / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-models-linux (mobilebert, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

This comment was automatically generated by Dr. CI and updates every 15 minutes.