You test your engines while you develop them, but once they are production ready, you don't need to test them that much. But keeping the lights on means doing maintenance of the stands, tanks, etc.

NASA’s Space Launch System (SLS) Program is concluding its structural qualification test series with one upcoming final test that will push the design for the rocket’s liquid oxygen tank to its limits ...

NASA plans to begin testing RS-25 engines for its new Space Launch System (SLS) in the fall of 2014, and the agency’s Stennis Space Center in Mississippi has a very big — literally — item to complete ...