chore: reorganize test files into domain-specific subdirectories

- 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

test/number/divide.wren

@@ -0,0 +1,5 @@
+io.write(8 / 2)        // expect: 4
+io.write(12.34 / -0.4) // expect: -30.85
+
+// TODO(bob): Unsupported RHS types.
+// TODO(bob): Divide by zero.