Add missing unit tests (python code)

[Apollys]

All nontrivial functions should have a unit test.

Missing unit tests:

• [ ] file_helper.py is missing unit tests
• [ ] loot_filter_rule_test.py is incomplete (see TODO in code)
• [ ] parse_helper_test.py is very incomplete (see TODO in code)
• [ ] string_helper.py is missing unit tests