Only run system tests as root in developer mode
Running system tests with root privileges is potentially dangerous.
Only allow it when explicitly requested by building with
--enable-developer.
Background:
- https://support.isc.org/Ticket/Display.html?id=16783
- https://mattermost.isc.org/isc/pl/yy7auk73rifm3x4pgee58sgccy
Instead of further tweaking the UID switching test (which is tedious to
get right because of all the "host scenarios" we need to account for),
prevent it from being run altogether unless --enable-developer is
used.
Edited by Michał Kępień