add llm/export to pytest.ini

[lucylq]

Summary

So the test runs in CI

Test plan

python -m unittest extension/llm/export/test_export_passes.py 
....
----------------------------------------------------------------------
Ran 4 tests in 0.696s

OK

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/8502

• 📄 Preview Python docs built from this PR

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

✅ You can merge normally! (1 Unrelated Failure)

As of commit 569aff2 with merge base 7964bca ():

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

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

• pull / android / run-emulator (gh) (trunk failure)
  The process '/usr/bin/sh' failed with exit code 1

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