[Backend Tester] Update README

[GregoryComer]

Summary

Update the readme for the backend test suite to describe how to run with pytest and to generally update for recent changes. Add CLI examples for common invocation patterns (filter by test, flow, or backend) and add some brief info on the JSON report format.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14739

• 📄 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, 4 Pending

As of commit bd26581 with merge base b1309e7 ():

NEW FAILURE - The following job has failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t c7e33e3a21e2d1da01992ceefdc488015f74f027475ace7bd1f7ea87ddf78a9d /exec failed with exit code 1

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

[psiddh]

@pytorchbot cherry-pick --onto release/1.0 -c docs

[pytorchbot]

Cherry picking #14739

Command git -C /home/runner/work/executorch/executorch cherry-pick -x -X theirs e608a21fc1845d960142b4c78ade06cdafdf5036 returned non-zero exit code 1

The previous cherry-pick is now empty, possibly due to conflict resolution.
If you wish to commit it anyway, use:

    git commit --allow-empty

Otherwise, please use 'git cherry-pick --skip'
On branch cherry-pick-14739-by-pytorch_bot_bot_
You are currently cherry-picking commit e608a21fc1.
  (all conflicts fixed: run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

nothing to commit, working tree clean

Details for Dev Infra team

Raised by workflow job