Is a manufacturer who tests a component to determine the extent of needed repairs or to determine the serviceability required to be covered under a drug and alcohol program when performing work for a 14 CFR Part 121/135 air carrier?

Yes, because the testing is being performed to a standard required by the manufacturer or other standards acceptable to or approved by the Federal Aviation Administrator.  The testing standard may be part of an inspection requirement in the technical data being used in the testing process.