ipatests: use pexpect to invoke ktutil
    
    `ktutil` is a REPL-style utility that can be controlled only interactively.
    The common approach of sending commands to stdin does not work with it on
    systems where `readline` library has version less then 8.0 due to a bug
    in that version.
    With `pexpect` we avoid this bug because it emulates the terminal
    when interacting with spawned process instead of simply sending all input
    to stdin.
    
    Related to: https://pagure.io/freeipa/issue/8690
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>