test_parameters: Ignore specific error message
    
    In Python 3, the error message from the decimal module is
    less clear than before.
    (It's apparently the price to pay for speed -- Python3 uses
    libmpdec as its Decimal implementation by default.)
    
    Don't check for the exact error message.
    
    https://fedorahosted.org/freeipa/ticket/5638
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>