MacOS, broken on "py.test -v"

'tf.random_normal' broken on  MacOS Mojave, Intel(R) HD Graphics 630 and ATI Radeon Pro 560
<img width="1385" alt="schermata 2018-11-28 alle 23 44 49" src="https://user-images.githubusercontent.com/32539407/49187382-99a97200-f367-11e8-892d-317bbd8f1ce3.png">
[clinfo.txt](https://github.com/hughperkins/tf-coriander/files/2626862/clinfo.txt)

