tests: Fix previous commit for selinux testing
tests: Fix previous commit for selinux testing

I only checked the test passed, I didn't read the output closely, and
made it succeed without testing anything.

Fix the absolute/relative `/etc` references.

Closes: #937
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree