VolumeMaterialMapperTests uses true random numbers

I noticed that VolumeMaterialMapperTest uses random numbers like

std::random_device               rd;
std::mt19937                     gen(rd());

which we should never do in our unit tests.

@fklimpel Can you fix the true random numbers, and the underlying failure?

Edited Apr 15, 2019 by Paul Gessinger-Befurt

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