Add several tests to test QA:Testcase_partitioning_custom_btrfs_preserve_home.
    
    This PR adds the `install_btrfs_upload` to install the btrfs based
    image, the `btrfs_preserve_home_extras` to prepare and test the data
    on the home partition, as well as the `custom_btrfs_preserve_home` that
    uses the preinstalled btrfs image and uses its current partitioning to
    preserve the home partition and the data on it.