Aggresive scan chain masking for improved diagnosis of multiple scan chain failures

When multiple chains, mapped to the same compactor output fail, AND-gate masking logic at the compactor side can be used to aid in diagnosis. The basic idea is: if for some pattern, only one faulty chain is observed and all the other faulty chains are masked, then the corresponding compacted response will only be affected by the non-masked faulty chain. Such a test pattern will help to diagnose that chain.