[autotests] Try to detect whether distro calls cursor theme DMZ-White or Vanilla-DMZ

Some distributions (e.g. Arch, FreeBSD) call the DMZ-White cursor theme
Vanilla-DMZ. Due to that our tests are failing even if the correct theme
is installed (see also T6623). This change tries to detect whether
DMZ-White is installed by looking into the GenericDataLocation. If not
found we set to Vanilla-DMZ. No guarantee that the check works for all
setups, but it's only tests...