Fix spilling test assertion that fails on rare occasions

[pentschev]

See link below for an example:

https://gpuci.gpuopenanalytics.com/view/rapids-branch-gpu-matrix/job/gpuCI/job/dask-cuda/job/branches/job/dask-cuda-gpu-matrix-branch-0.8/CC_VERSION=5,CUDA=9.2,LINUX_VERSION=ubuntu16.04,PYTHON=3.6/37/console

cc @raydouglass

[pentschev]

It has failed recently again:

https://gpuci.gpuopenanalytics.com/job/docker/job/tests/job/docker-test-daskcuda/CUDA_VERSION=10.0,LINUX_VERSION=ubuntu18.04,PYTHON_VERSION=3.7/76/consoleFull

cc @rlratzel

[pentschev]

#145 is another example of the failure reported here.

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[github-actions]

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