Add test covering the guided-shrink testcase. This will require both in-install - to walk through the partition shrinking - and post-install - to check whether the data on the shrunk partition is intact.

Check that the hdd-images created by the createhdds.sh are usefull for the testcase, and either change one of the existing, or create a new one if not.

There is a strong possibility that the post-install steps will be same as in disk_guided_free_space_postinstall.pm.