tests/integration: fix running with systemd 251
udev/systemd, starting version 251 (a791061) started to verify that paths are actually on sysfs, i.e. the fs type is SYSFS_MAGIC. Since we use umockdev to emulate sysfs on normal file system this check will now fail. There is an exit hatch in form of an environment variable called SYSTEMD_DEVICE_VERIFY_SYSFS
that when set to 0
makes systemd/udev forgo that check. umockdev >= 0.17.10 does set that variable itself but in Python changes to the env by libraries does not effect the env of the running script. Thus we have to set it ourselves.