The tests are stored in results/YY/MM/DD so you can nuke directories as needed. You can also set up archiving rules in settings.ini and set up cron to run cli/archive.php
At some point I need to add code to prune tests automatically based on available disk spaceas an option.
I think with a tiny change to allow for “none” or “/dev/null” as the archive location we could make it work out of the box as well without having to hack the script. Only reason it won’t work right now is that it verifies that the archive worked before deleting but we could special-case /dev/null and skip the archiving (probably best to have that logic inside archive.inc proper so it also doesn’t try to restore).