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/minus.wren

@@ -0,0 +1,10 @@
+// Infix.
+io.write(5 - 3)      // expect: 2
+io.write(3.1 - 0.24) // expect: 2.86
+io.write(3 - 2 - 1)  // expect: 0
+
+// Unary negation.
+var a = 3
+io.write(-a) // expect: -3
+
+// TODO(bob): Unsupported RHS types.