Based on an IRC conversation with @SubrahamanyamVarma, turning this into a task.
scandium currently runs Stretch (like the Parsoid servers itself). scandium in addition also hosts testreduce, which is written in Nodejs (unlike current Parsoid, which is PHP). The Parsoid parts need to remain on Stretch as long as the production Parsoid servers run Stretch.
scandium runs the node10 component and npm from stretch-backports, but stretch-backports is going away soon and the last version on stretch-backports is no longer compatible with the node10 component, various dependencies clash.
restreduce does not need to run on the same host like Parsoid. Given that Buster has Node 10 by default and npm as well, we should create a separate Ganeti instance for testreduce (and then we can reimage scandium to get a clean Stretch state again (or we remove all Node traces)).
Potentially this would also allow the visual-diff tests to run on the new Ganeti instance, but we need to check the performance characteristics.