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/list/add.wren

@@ -0,0 +1,10 @@
+var a = []
+a.add(1)
+io.write(a) // expect: [1]
+a.add(2)
+io.write(a) // expect: [1, 2]
+a.add(3)
+io.write(a) // expect: [1, 2, 3]
+
+// Returns added element.
+io.write(a.add(4)) // expect: 4