Fix CPUObserver test

Summary: CPUObserver observe events happen in different threads than the IOThreadPool ones.

Test Plan: fbconfig folly/wangle/concurrent/test && fbmake runtests --stress-runs 48

Reviewed By: jsedgwick@fb.com

Subscribers: doug, fugalh, folly-diffs@

FB internal diff: D1769345

Tasks: 5948465

Signature: t1:1769345:1420654316:196121ca30c905497a0acade05b9d877c98f6eeb

folly/wangle/concurrent/test/ThreadPoolExecutorTest.cpp

@@ -337,7 +337,7 @@ class TestObserver : public ThreadPoolExecutor::Observer {
     ASSERT_EQ(threads_, 0);
   }
  private:
-  int threads_{0};
+  std::atomic<int> threads_{0};
 };
 
 TEST(ThreadPoolExecutorTest, IOObserver) {