test: use setpriv instead of su for user switch from root

test: use setpriv instead of su for user switch from root

systemd-repart needs to find mkfs.ext4 for the test.
This is located in the directory /usr/sbin on openSUSE Tumbleweed.
But since the variable ALWAYS_SET_PATH in /etc/login.defs is set to yes,
su re-initializes the $PATH variable and removes /usr/sbin.
Hence, mkfs.ext4 is not found and the test fails.
Using setpriv instead of su fixes this issue and is more appropriate to
do the switch user task from root.

[zjs: move setpriv to $BASICTOOLS and force-push to retrigger CI]