Quibble jobs currently run on stretch, which means they're stuck in Chromium 73, which is rapidly getting out of date.
Buster keeps getting new versions of Chromium (currently 80), so moving to that fixes this problem.
However, we don't want to move the main phpunit/qunit tests away from stretch, as that's what we're running in Wikimedia production.
Consequently, we'll split all repos into selenium and non-selenium jobs, and have the selenium job run on buster, unlike the non-selenium one.