Orange County supervisors unanimously approved a plan for drug and alcohol testing of applicants to be county firefighters, law enforcement officers and heavy equipment operators. The tests, said the plan's sponsor, Supervisors Chairman Harriett M. Wieder, will begin as soon as a contract can be negotiated with a firm to administer them. "As a major employer with public health and drug enforcement responsibilities, we need to take this step," Wieder said before the board vote. She cited a 1985 study that showed drug use among Californians to be 2.5% higher than the national average in certain age groups. Wieder said there are no current plans to extend the testing beyond applicants for "safety jobs."