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/assignment/syntax.wren

@@ -0,0 +1,11 @@
+// Chained assignment.
+var a = "a"
+var b = "b"
+a = b = "chain"
+io.write(a) // expect: chain
+io.write(a) // expect: chain
+
+// Assignment on RHS of variable.
+var c = a = "var"
+io.write(a) // expect: var
+io.write(c) // expect: var