Backport upstream tests and fixes
    
    Commits backported:
    
    * Add tests for LUKS binding and unbinding
    - https://github.com/latchset/clevis/pull/134/commits/f5d42cb3ba02ff8fcf39cc6c2751d2cd444f5a74
    
    * Rework the logic for reading the existing key
    - https://github.com/latchset/clevis/pull/141/commits/834eda9db62992fc235ecef41461502fd4084f6e
    
    * fix for different output from 'luksAddKey' command w/cryptsetup v2.0.2 (
    - https://github.com/latchset/clevis/commit/62bd6de0b8c36a238809546273ff08c3f3766564
    
    * pins/tang: check that key derivation key is available
    - https://github.com/latchset/clevis/pull/119/commits/c231352729223b08ade04e4e46c775bb934c9959