tests: aux.sh limit device

tests: aux.sh limit device

During test do a more close selection of visible devices.
If some test leaks a device with LVMTEST prefix, next
test should not be influnced (or parallel running one).

If the test is running in non-/dev  dir - it's already protected
by full path with $PREFIX in it - however it test is running
in real /dev dir - there was no such protection and test
were confused when they have seen such leaked device.