Fix flaky test in dataset_ops_test.py
Fix flaky test in dataset_ops_test.py

Summary:
```
while pytest caffe2/python/operator_test/dataset_ops_test.py::TestDatasetOps::test_collect_tensor_ops; do sleep 0.1; done
```
Run this long enough and you'll see an error like this:
```
Sample histogram: [ 92 109  65 103  99 104  99 125 100 104]
...
>       self.assertTrue(all(hist > 0.7 * (num_to_collect / 10)))
E       AssertionError: False is not true
```
I've seen values like 65, 68, 69, 70. Setting the cutoff at 60 instead of 70 seems safe enough.

/cc Yangqing (or whoever authored https://github.com/caffe2/caffe2/commit/a56b881c4a4fd8a4e7554d0b31e6e0326d26e718).
Closes https://github.com/caffe2/caffe2/pull/840

Differential Revision: D5292120

Pulled By: akyrola

fbshipit-source-id: 2ea4cbb58e206268759bd9d3639e8921623f519c

bwasti/caffe2,xzturn/caffe2,caffe2/caffe2,bwasti/caffe2,pietern/caffe2,sf-wind/caffe2,sf-wind/caffe2,Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2,davinwang/caffe2,xzturn/caffe2,Yangqing/caffe2,bwasti/caffe2,xzturn/caffe2,davinwang/caffe2,Yangqing/caffe2,pietern/caffe2,sf-wind/caffe2,sf-wind/caffe2,xzturn/caffe2,davinwang/caffe2,bwasti/caffe2,pietern/caffe2,pietern/caffe2,davinwang/caffe2,sf-wind/caffe2,bwasti/caffe2,pietern/caffe2,Yangqing/caffe2,davinwang/caffe2