A fair amount of the time, I want to test a change to a maintenance script or to export/import-related functionality in a more production-like environment, after having tested it to the degree possible on my laptop. In some cases colleagues want to test such changes which require the setup of pretty complicated extensions, that are already set up and configured in deployment-prep so that we don't have to.
Unfortunately, the process for testing on an existing instance is fairly painful, given the upstream sync that wipes out one's changes every 10 minutes, and that adding phpunit to an instance can't be done with existing packages.
We (or I) could set up a dedicated instance that would have the appropriate phpunit phar files plus a change to CommonSettings-labs including those if they are found on an instance. This instance would not be included in mediawiki cap targets so it would not receive the upstream sync automatically. A big fat warning would be added to the MOTD reminding people who use the instance to scap pull before they run anything (including copying over their changes for testing). This host would look more or less like an mwmaint instance but without any cron jobs, and of course not being in the pool to serve mediawiki requests.
This task serves as a place to start the discussion with releng and whomever else has ideas about it.