udev-test: make sure we run udev tests with selinux assumed off

udev-test: make sure we run udev tests with selinux assumed off

This is cleaner that way given that we create our own half-virtualizes
device tree, and really shouldn't pull selinux labelling and access
control into that, we can only lose, in particular as our overmounted
/sys/ actually lacks /sys/fs/selinux.

(This fixes udev test woes introduced by #16821 where suddenly the test
would fail because libselinux assumed selinux was on, but selinuxfs
wasn't actually available)