community/doctest: move from testing

I've maintained doctest in testing/ since 2020-07-03, when it was checked in at version 2.4.0. While there has only been one release since then, I quickly brought the package up to date in testing/, and can commit to maintaining this package in the future. I request that it be moved to community/ from testing/, where it will serve as a dependency of Notcurses, which I've likewise requested to move to community/ in #14576 (closed).