>Or do they test all...

Depends what specification you use. There will always be infant mortality 
failures that give you the leading edge of the "bathtub curve", and for the 
various MIL-SPEC devices these are filtered out at various stages, and the 
higher spec you go to, the more likely they are to have been removed from 
the mix by visual inspection, high temperature burn-in, or some other 
criteria. 

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