test: fix a trivial typo in libtests.sh

When running the test-admin-deploy-1.sh unit test,
cat shows the following error:

cat: boot/vmlinuz-3-6.0: No such file or directory

due a trivial typo in the kernel image file name.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>

https://bugzilla.gnome.org/show_bug.cgi?id=706371