Fix accuracy test for MNIST CNN with IOType=io_parallel and Strategy=resource

[jmduarte]

Originally posted by @thesps in #362 (comment)

FAILED test_cnn_mnist.py::test_accuracy[resource-io_parallel] - assert (0.9885 > 0.95 and 0.8867981790591806 < 0.01)
test_cnn_mnist.py fails for IOType=io_parallel and Strategy=resource.

[jmduarte]

Should be solved with #448