Use the Calamares installer and test whether the image still works

For every live image, the Calamares installer is started after being booted with BIOS, UEFI and secure UEFI. Then the resulting disk image is booted and checked whether it basically functions.

Special things to consider when reviewing:

• I've used sudo install-debian to avoid a authentication prompt. It works fine, but does not emulate what a regular user would do.
• After the installer is finished, the conversion of the resulting hard disk takes a long time (at least 10 minutes) and about 5GB. With 9 images and 3 variants, this amounts to quite some additional disk space. Alternatively, the test steps after the installation could be integrated into the main test (similar to what SUSE is doing), or they can be marked as 'private assets' (which probably makes them eligible for deletion earlier)
• Interesting: in order to boot properly, after the installation finished, the PFLASH_VARS need to be migrated as well, otherwise the newly added UEFI variables would not be tranferred.
• Other variants that might be added at a later moment:
  • Click in the .desktop entry
  • Other scenarios with co-installation of Debian alongside e.g. Windows (see openSUSE https://openqa.opensuse.org/tests/overview?distri=windows&version=10&build=21H2&groupid=22)
  • UK keyboard layout :-)

I've marked the MR as draft, but if you are fine with it, you can merge it already.