Add unit tests for the GPU metrics

[amete]

This can be optional but we should eventually add unit tests for the GPU metrics. Currently we have no coverage on that front. Now, this is a bit more involved I suppose because we possibly need to add a bit of cuda code and make our cmake setup smarter to be able to build it via nvcc etc. This is definitely not a showstopper but would be good to address sooner than later.

[graeme-a-stewart]

Yep, we should do this and at least check that we get some metrics back from GPUs, so it will require some extra codes in CMake to do this (fortunately, NVIDIA GPUs are well supported in CMake, but it may bump the minimum version).

[graeme-a-stewart]

I wonder, could we say this is fixed with #195? It seems a very smart way to test the monitor without actually needing to depend on a graphics card itseld?

[graeme-a-stewart]

We condsider closed by #195