test: add tests for NetworkPolicyEngine audit export and clear (closes #2447)

[tmdeveloper007]

Summary

Adds unit tests for NetworkPolicyEngine.export_audit_log and
NetworkPolicyEngine.clear_audit_entries in backend.secuscan.network_policy.

Changes

• New test file: testing/backend/unit/test_network_policy_audit_export.py
• 11 tests covering: JSON export, CSV export, empty-log edge cases,
  clear entries, post-clear independence

Testing

• All 11 tests pass with python -m pytest testing/backend/unit/test_network_policy_audit_export.py --noconftest -v

[utksh1]

Request changes: this test module also mutates the module-level singleton without restoring it and hard-codes a temporary filesystem path for its audit file. Use the pytest temporary-path fixture plus a fixture that restores singleton state after each test. Parse CSV with Python csv instead of splitting on newlines, and remove the unused pytest import.