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/local.wren

@@ -0,0 +1,10 @@
+fn {
+  var a = "before"
+  io.write(a) // expect: before
+
+  a = "after"
+  io.write(a) // expect: after
+
+  io.write(a = "arg") // expect: arg
+  io.write(a) // expect: arg
+}.call