Fix ObjectSizeTrackingWithMutation unit test

Summary: The vector test failed opt mode starting today; it can still pass dbg and dev mode though https://www.internalfb.com/intern/test/562950043592554/. Very likely it's because opt mode did certain optimization these days.

Reviewed By: therealgymmy

Differential Revision: D48359277

fbshipit-source-id: 5e7b1d0573b90c5a58febe0822644d227382b637

Author: Jiayue Bao

cachelib/experimental/objcache2/tests/ObjectCacheTest.cpp

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+#include <folly/Benchmark.h>
 #include <folly/Random.h>
 #include <gtest/gtest.h>
 
@@ -634,6 +635,7 @@ class ObjectCacheTest : public ::testing::Test {
     ThreadMemoryTracker tMemTracker;
     auto memUsage1 = tMemTracker.getMemUsageBytes();
     auto objectCopy = std::make_unique<T>(*object);
+    folly::doNotOptimizeAway(objectCopy);
     auto memUsage2 = tMemTracker.getMemUsageBytes();
 
     EXPECT_EQ(memUsage2 - memUsage1, objcache.template getObjectSize(object));