Don't allow test suite to be run as non-root
Originally created by @anonym on #17613 (Redmine)
Since bc80e77e this is not allowed, but I have already heard someone missing that fact and ending up wasting time and getting frustrated. So let’s add a check, but one that can be disabled for those of us who really want to run it as non-root sometimes.
Feature Branch: test/17613-root-check