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/field/multiple.wren

@@ -0,0 +1,29 @@
+class Foo {
+  set(a, b, c, d, e) {
+    _a = a
+    _b = b
+    _c = c
+    _d = d
+    _e = e
+  }
+  write {
+    io.write(_a)
+    io.write(_b)
+    io.write(_c)
+    io.write(_d)
+    io.write(_e)
+  }
+}
+
+var foo = Foo.new
+foo.set(1, 2, 3, 4, 5)
+foo.write
+// expect: 1
+// expect: 2
+// expect: 3
+// expect: 4
+// expect: 5
+
+// TODO(bob): Inherited fields.
+// TODO(bob): Trying to get or set a field outside of a class.
+// TODO(bob): Fields in nested classes.