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/mod.wren

@@ -0,0 +1,14 @@
+io.write(5 % 3) // expect: 2
+io.write(10 % 5) // expect: 0
+io.write(-4 % 3) // expect: -1
+io.write(4 % -3) // expect: 1
+io.write(-4 % -3) // expect: -1
+io.write(-4.2 % 3.1) // expect: -1.1
+io.write(4.2 % -3.1) // expect: 1.1
+io.write(-4.2 % -3.1) // expect: -1.1
+
+// Left associative.
+io.write(13 % 7 % 4) // expect: 2
+
+// TODO(bob): Unsupported RHS types.
+// TODO(bob): Error on mod by zero.