[Macros] Add test that triggered a reference cycle.

Fixes rdar://108565923.

Author: DougGregor

test/stdlib/Observation/Observable.swift

@@ -10,6 +10,18 @@ import StdlibUnittest
 import _Observation
 import _Concurrency
 
+@available(SwiftStdlib 5.9, *)
+@MainActor @Observable
+final class StateMachine {
+  enum State {
+    case initializing
+    case running
+    case complete
+  }
+
+  var state: State = .initializing
+}
+
 @usableFromInline
 @inline(never)
 func _blackHole<T>(_ value: T) { }