Default GPU_COUNT to 1 in cmake file

[ChuckHastings]

Closes #2314

Rather than relying on the output from nvidia-smi, MG C++ tests will default to 1 GPU. This default can be overridden by the developer by calling cmake with the option: -DGPU_COUNT= and specifying the desired number of GPUs.

Note that the user can always run individual tests by directly calling mpirun. This only affects the default used by ctest, make test or ninja test to run the entire suite of tests.

[codecov-commenter]

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[rlratzel]

@gpucibot merge