Use black_box when running benchmarks to avoid compiler constant folding

[AngelicosPhosphoros]

No description provided.

[dragostis]

Thanks for the PR! black_box in these benchmarks doesn't seem to be needed since the result of the computation can't simply be discarded since it's used in the assert.

I'm going to close this PR, but feel free to re-open if you think that the current benchmarks are inaccurate in any way.