Add unit tests for genRuleFile.py

[rohithsiddi]

flow/util/genRuleFile.py is the most actively maintained Python utility in the repo and is used in CI via the update_rules Makefile targets. Despite this, it had zero test coverage. This PR adds 30 unit tests covering all rule calculation modes and edge cases.

Changes

• flow/test/test_genRuleFile.py: new test file with 30 tests across two classes

Coverage

• comma_separated_list() argument parser helper
• All 4 production rule modes: direct, padding, period_padding, metric
• Rounding behavior (round_value: true produces int, false produces float)
• Wildcard pattern matching (*flow__warnings__count:*)
• Old rules interaction: --tighten, --failing, --update modes
• Edge cases: string metric fields skipped, missing/multiple clocks warnings
• metrics_to_consider filtering (preserves old rules for unlisted metrics)
• CTS buffer special case (max(rule_value, metric * 1.1))
• min_max with min_max_direct option

Test plan

• cd flow/test && python3 test_genRuleFile.py -v (30/30 pass)
• All existing util tests still pass (no regressions):
  • test_convertDrc.py (7 tests)
  • test_def2stream.py (14 tests)
  • test_genElapsedTime.py (5 tests)
  • test_generate_klayout_tech.py (13 tests)