Nested virt is not reliable enough, especially on other arches, to rely
on for testing the created images. This moves the test code into
test_boot_* scripts to be run from inside the booted images.
It also adds copying the results of the build into
/var/tmp/test-results/, and includes the generated ssh key so that
whatever boots the image can also log in.
The tests/test_image.sh script has been added to handle running the
test_boot_* scripts without any of the extra lorax-composer specific
setup.
Cherry-picked from b8bf258a3c
Related: rhbz#1769525
