Could you add a similar test please?
In short: run a command using root_cmd that is supposed to return no output (no stdout, no stderr, all empty). For example:
root_cmd test -d /usr 2>&1
But if there is any stdout or stderr, then that’s a system configuration issue unrelated to Whonix Linux Installer and installation should be aborted.
(Came up in in context of (Fixed) Build From Source Error: special device /dev/mapper/to does not exist)