Tests: harden ruamel detection and gate fixture generators

[a-v-popov]

A few items I'd suggest addressing before merging PR #3345, plus two small follow-ups:

1. The detection probe matches the namespace shim, not the library

tests/utils.py does import ruamel, but ruamel is a PEP 420 namespace package — it can survive uninstalls of ruamel.yaml (the ruamel/ directory often lingers, or another package owns part of the namespace). On my system right now:

>>> import ruamel
>>> import ruamel.yaml
ModuleNotFoundError: No module named 'ruamel.yaml'

HAS_RUAMEL becomes True while Box/PyYAML disagree, and clean_ruamel runs for nothing. python-box itself probes with from ruamel.yaml import version_info, YAML (box/converters.py:22). Suggested:

try:
  import ruamel.yaml  # noqa: F401
  HAS_RUAMEL = True
except ImportError:
  HAS_RUAMEL = False

2. yaml.dump should pin default_flow_style=False

Box.to_yaml(...) in python-box 7.x defaults to default_flow_style=False, width=120 and calls yaml.dump(obj, default_flow_style=default_flow_style, width=width, ...) (box/converters.py:195, box/box.py:1018). The PR replaces it with:

return yaml.dump(topology.to_dict(), width=120)

PyYAML's own default for default_flow_style is None ("auto: flow for inner scalar-only collections"), not False. Today's fixtures are all under-mappings, so the two modes coincide and the suite passes. The next fixture with a top-level scalar list, or a deeply nested scalar-only structure, will silently diverge from the format every committed expected/*.yml was generated in. One-line fix:

return yaml.dump(topology.to_dict(), default_flow_style=False, width=120)

3. log._ERROR_LOG = [] is redundant

The previous line was log.err_count = 0, which was a typo from the start — log.err_count doesn't exist; the public counter is get_error_count() at netsim/utils/log.py:389. The new log._ERROR_LOG = [] does target a real attribute, but run_test immediately calls log.init_log_system(header=False), which itself does _ERROR_LOG = [] (log.py:539). The reset is redundant, and reaching into a leading-underscore symbol from outside the module is a smell. Either drop the line, or call log.init_log_system(header=False) if a defensive reset is desired.

Two small follow-ups worth folding in

• tests/conftest.py (new): the standard-pytest-safe way to surface "ruamel.yaml is installed, things will be slower" is pytest_configure + config.issue_config_time_warning(UserWarning(...)). It appears in pytest's warnings summary, never fails a run, and doesn't touch collection / -k filtering / parallelism. Import HAS_RUAMEL from tests/utils.py so there's a single source of truth.
• Gate the fixture generators. The PR docs already say error-log fixtures can't be generated under ruamel, but tests/create-error-tests.sh doesn't enforce it — a user can still run it and commit broken .log files. A 4-line python3 -c "import ruamel.yaml" && exit 2 guard at the top closes that hole. tests/create-transformation-test-case.py does work under ruamel (the clean_ruamel walk handles it), so a stderr advisory there is enough.

Reference implementation of all of the above on a-v-popov/netlab@pr-3353-followup (compare view) — verified end-to-end:

• Without ruamel.yaml: xform + error tests pass; no warning.
• With ruamel.yaml 0.19.1: xform + error tests pass (+~40% wall time, matching the "slower" claim); UserWarning appears in pytest's warnings summary; nothing fails or skips.
• create-error-tests.sh with ruamel.yaml installed: exits 2 with an actionable message before doing anything destructive.
• ruff check . clean.