One of the three main engines scheduled to power the space shuttle Discovery on the first post-Challenger mission may have sprung a leak during a test-firing, a NASA spokesman said. Ed Medal of the Marshall Space Flight Center in Huntsville, Ala., which supervised the tests, said if there is a leak, a spare engine would be used for the flight. He said using a spare might cause a delay in engine delivery to the Kennedy Space Center launch site in Florida, but he did not know if the targeted launch date of June 2, 1988, would be affected. Medal said a 520-second firing of the liquid fuel engine on Saturday in Mississippi appeared normal but a detailed checkout of the power plant indicated a potential leak in the oxidizer heat exchanger. He said engine inspection is continuing.