On an exact length value of a minimal single diagnostic test for one class of circuits

We consider a problem of synthesis of irredundant logic circuits in the basis $\{\&,\lor,\lnot\}$ which realize Boolean functions on $n$ variables and allow short single diagnostic tests regarding uniform constant faults on outputs of gates. For each Boolean function permitting realization by an irredundant circuit, the minimal possible length value of such a test is found. In particular, it is proved that this value does not exceed two.