tests: Fix U-Boot test case

commit 08b873457 ("deploy: Remove checksum from generated loader entries")
changed the generated loader entries file names thus making the U-Boot test
case to fail since this test parsed those files.

Fix test-admin-deploy-uboot.sh by looking to the updated files names.

https://bugzilla.gnome.org/show_bug.cgi?id=708511

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

It needs to be installed so it'll be run by the InstalledTests scheme.