Add test for coro::timeout on final asyncgenerator result

Summary: This was broken before D29680441 (https://github.com/facebook/folly/commit/832f135adede09d6e83e1f026c159d1b88249951)

Reviewed By: afernandez90

Differential Revision: D29913022

fbshipit-source-id: 5fa370a0ec3285875073ee84898601a41fff55e2

folly/experimental/coro/test/TimeoutTest.cpp

@@ -179,6 +179,11 @@ TEST(Timeout, AsyncGenerator) {
                     .next(),
         1s));
     EXPECT_TRUE(tryResult.hasException<std::runtime_error>());
+
+    // Generator completing normally.
+    result = co_await coro::timeout(
+        []() -> coro::AsyncGenerator<int> { co_return; }().next(), 1s);
+    EXPECT_FALSE(result);
   }());
 }