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

Originally posted by @thesps in https://github.com/fastmachinelearning/hls4ml/issues/362#issuecomment-897581793

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.