New optional relative benchmark for astropy PR

[Pey Lian Lim]

Many thanks to Mridul Seth who implemented https://github.com/astropy/astropy/pull/15779 that allows a PR in astropy to run benchmarks from https://github.com/astropy/astropy-benchmarks/tree/main/benchmarks when a "benchmark" label is applied to the PR by a maintainer. However, we can only benchmark what is actually, uh, benchmarked, so YMMV. FYI.