Add test file for Foo class constructors covering zero, one, and two argument overloads by arity. Verify each overload produces correct output and that new instances return proper type and toString behavior.
- Moved unit tests for user authentication into `tests/unit/auth/`
- Relocated integration tests for payment processing to `tests/integration/payments/`
- Grouped API endpoint tests under `tests/api/` with separate files for each resource
- Updated test runner configuration to discover tests in new directory structure
Bob Nystrom