setupTestServer built the settings registry (and wired it into
systemSettingsHandler) but never called authHandler.SetSettings, unlike
main.go:112. The auth handler therefore ran with a nil registry and
defaultTimezone() took its nil-fallback branch, returning "UTC" for every
Register/AdminCreateUser in tests regardless of the system default_timezone
setting — so the 740213f0 inheritance pin in TestAdminCreateUser could never
pass and failed every full-suite run.
Also replace the TestAdminCreateUser "stable-ish" uniquifier (MaxConns is
pinned to 1 by the harness, so it produced the same user every run and
depended on the next run's cleanup sweep) with a real uuid suffix.