Forcibly install syslinux* from Jessie, instead of our Wheezy backport.
Explicitly install syslinux-efi.

For some reason, it's not pulled anymore in our Jessie ISO, which breaks
the build.
Also forcibly pull isolinux from Jessie.

Otherwise:

The following packages have unmet dependencies:
 syslinux-common : Breaks: isolinux (< 3:6.03~pre20+dfsg-4) but
 3:6.03~pre20+dfsg-2~bpo70+1 is to be installed
Adjust syslinux.efi path to match the one in syslinux-efi 3:6.03~pre20+dfsg-4.
Adjust test suite setup instructions for Jessie.