Also build for libcxx 18; add an unconstrained build

[h-vetinari]

As promised in #9:

However, to not turn this into a bootstrapping nightmare (e.g. new libcxx needing a libcxx-testing that pins a version that doesn't exist yet), I'll re-add an escape-hatch where the main branch of libcxx will not pin the patch version of libcxx-testing, and get a libcxx-testing without a libcxx constraint.

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.