Excluding non-zero exit codes from Relative Time comparison.

I usually run a few sets of parameter lists which quickly explodes in number of instances (which is working as intended!). However, all the runs are shown in the summary at the end including those that exited with an non-zero exit code.

Looking at the code base, adding a filter in src/benchmark/relative_speed::compute to remove results with non-zero exit codes from the output should fix it.

I'm not really sure if there are more dependencies on the compute function elsewhere though or I would have just pushed a PR.

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PS. Thanks for the all the great software! I use hyperfine, fd, and bat all the time!