Fix timing issue in stats_test.cc
Fix timing issue in stats_test.cc

Summary:
Failure in https://ci.pytorch.org/jenkins/job/caffe2-builds/job/py2-mkl-ubuntu16.04-test/135/
Closes https://github.com/caffe2/caffe2/pull/1629

Reviewed By: azzolini

Differential Revision: D6581743

Pulled By: pietern

fbshipit-source-id: 8c84f8c959015d7717785ee3f37b93c4ef146f96

Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2,caffe2/caffe2,Yangqing/caffe2