The test is to make sure that the insulation can withstand a constant stress for a specified lenght of time without bresking down.
The main reason for using DC to test insulation is that DC presents a constant stress on the insulation material. AC is, of course, a constantly varying stress, which allows the insulation to recover to some extent when the voltage falls into the trough of the waveform. A DC stress is constant, and will not allow the stress on the insulation to recover until the stress is removed or it breaks down.
Dave M