tests_py3: decode get_file_contents() result
    
    When running tests in python3 we get bytes object instead of
    bytestring from get_file_contents() and when passing it to
    run_command() we later fail on concatenation in shell_quote().
    
    https://pagure.io/freeipa/issue/7131
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>