* Addon Manager: Refactor utilities tests to remove filesystem use
* Addon Manager: Move process_date_string_to_python_datetime to utilities
Also add unit tests and modify the exception type
* Addon Manager: Add tests for other date separators
* Addon Manager: Refactor to reduce duplication
* Addon Manager: add explanation of why the function exists
* Addon Manager: use exception chaining
* Addon Manager: Remove unused test files