Add custom partitioning tests for blivet
    
    Summary:
    This adds custom partitioning tests for testing blivet-gui. I've added
    new function to anaconda library, because it doesn't make much sense adapting existing
    helper functions (e. g. `custom_change_type` or `custom_change_fs`), because of how
    differently blivet-gui works. I'm not running these tests on UEFI, because one needs
    to add all partitions by hand (UEFI needs one additional partition and there is currently
    a bug in blivet-gui where new partitions are added at the beggining of disk rather then
    at the end). This is still missing BTRFS and iSCSI tests, because I need to ask vtrefny
    what needs to be done in blivet-gui to replicate behaviour of old partitioning systen.
    
    Test Plan: Run all new tests.
    
    Reviewers: adamwill, jskladan
    
    Subscribers: tflink
    
    Maniphest Tasks: T938
    
    Differential Revision: https://phab.qa.fedoraproject.org/D1188