chore(librarian): preserve test_iam_logging.py (#14630)

`tests/unit/gapic/iam_logging_v1/test_iam_logging.py` is a handwritten
file and needs to be explicitly preserved


https://github.com/googleapis/google-cloud-python/blob/main/packages/google-cloud-iam-logging/tests/unit/gapic/iam_logging_v1/test_iam_logging.py

This PR resolves the following stack trace in the last `librarian
generate` job

```
nox > Running session unit-3.13(protobuf_implementation='upb')
nox > Creating virtual environment (virtualenv) using python3.13 in .nox/unit-3-13-protobuf_implementation-upb
nox > python -m pip install mock asyncmock pytest pytest-cov pytest-asyncio -c /repo/packages/google-cloud-iam-logging/testing/constraints-3.13.txt
nox > python -m pip install -e . -c /repo/packages/google-cloud-iam-logging/testing/constraints-3.13.txt
nox > py.test --quiet --junitxml=unit_3.13_sponge_log.xml --cov=google --cov=tests/unit --cov-append --cov-config=.coveragerc --cov-report= --cov-fail-under=0 tests/unit
/repo/packages/google-cloud-iam-logging/.nox/unit-3-13-protobuf_implementation-upb/lib/python3.13/site-packages/coverage/control.py:946: CoverageWarning: No data was collected. (no-data-collected); see https://coverage.readthedocs.io/en/7.10.7/messages.html#warning-no-data-collected
  self._warn("No data was collected.", slug="no-data-collected")

- generated xml file: /repo/packages/google-cloud-iam-logging/unit_3.13_sponge_log.xml -
no tests ran in 0.03s
nox > Command py.test --quiet --junitxml=unit_3.13_sponge_log.xml --cov=google --cov=tests/unit --cov-append --cov-config=.coveragerc --cov-report= --cov-fail-under=0 tests/unit failed with exit code 5
nox > Session unit-3.13(protobuf_implementation='upb') failed.
Traceback (most recent call last):
  File "/app/./cli.py", line 811, in _run_nox_sessions
    _run_individual_session(nox_session, library_id, repo)
  File "/app/./cli.py", line 836, in _run_individual_session
    result = subprocess.run(command, text=True, check=True)
  File "/usr/local/lib/python3.9/subprocess.py", line 528, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['nox', '-s', "unit-3.13(protobuf_implementation='upb')", '-f', '/repo/packages/google-cloud-iam-logging/noxfile.py']' returned non-zero exit status 1.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/app/./cli.py", line 994, in handle_build
    _run_nox_sessions(library_id, repo)
  File "/app/./cli.py", line 814, in _run_nox_sessions
    raise ValueError(f"Failed to run the nox session: {current_session}") from e
ValueError: Failed to run the nox session: unit-3.13(protobuf_implementation='upb')

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/app/./cli.py", line 1419, in <module>
    args.func(librarian=args.librarian, repo=args.repo)
  File "/app/./cli.py", line 996, in handle_build
    raise ValueError("Build failed.") from e
ValueError: Build failed.
```

Author: parthea

.librarian/state.yaml

@@ -2167,6 +2167,7 @@ libraries:
       - scripts/client-post-processing
       - samples/snippets/README.rst
       - tests/system
+      - tests/unit/gapic/iam_logging_v1/test_iam_logging.py
     remove_regex:
       - packages/google-cloud-iam-logging
     tag_format: '{id}-v{version}'