tests: fix mocking krb5_creds in test_copy_ccache
    
    To just test some ccache related functionality without talking to an
    actual KDC to get the tickets some needed libkrb5 structs were mocked
    based on tests from the MIT Kerberos source code. One struct member
    (is_skey) was so far not regarded by libkrb5 for out test case. But a
    recent fix for http://krbdev.mit.edu/rt/Ticket/Display.html?id=8718
    changed this and we have to change the mocking.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>