Toyota Motor Corp.’s small-car unit, Daihatsu Motor Co., said Thursday it will resume operations at all its assembly factories in Japan on May 7, some four months after it suspended its entire domestic production in late December due to safety test rigging. The automaker will resume production of the Copen minivehicle at its factory in Ikeda, Osaka Prefecture, which is the last of its four assembly factories in Japan to remain halted. Its factories in Kyoto, Shiga and Oita prefectures have already restarted operations, and the Oita plant is expected to ramp up production on May 6. The resumpti…