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/variable/local_in_middle_of_block.wren

@@ -0,0 +1,14 @@
+class Foo {
+  bar {
+    var a = "a"
+    io.write(a) // expect: a
+    var b = a + " b"
+    io.write(b) // expect: a b
+    var c = a + " c"
+    io.write(c) // expect: a c
+    var d = b + " d"
+    io.write(d) // expect: a b d
+  }
+}
+
+Foo.new.bar