print problems in deterministic order
    
    Currently the acceptance test suite is intermittently failing because
    the problems are being printed in a different order than expected.
    
    The actual order in which the problems are printed is irrelevant, so
    let's just sort the string messages before returning them, so that we
    always have a deterministic order.
    
    Change-Id: I952119b9e15e4abedd16aa7c2e2b638ae904ef54