By considering test costs at behavioral level, test problems can be pointed out during the first phases of the design flow. Thus, in case either some testability problems are identified or the size (and hence the cost) of the test set results to be too high, the designer or the high level synthesis tool can modify the circuit to reduce such testability problems. The main problem is the correspondence between the behavioral and RT or gate level fault models. To overcome such limitation, the paper presents a design flow based on the behavioral fault model modification ("evolution") depending on the actual RTL implementation.