For each test, there should be a US version and a completely different Europe and Asia version. Each test should be given at the same time everywhere it is given. So the US test would be given at 8 AM in Hawaii and 1PM in New York. The Europe and Asia test would be given at 8 AM in Britain and 4 PM in Japan.
This would eliminate the problem of students in Asia and elsewhere getting prior information about the problems and answers on the exams. Obviously recycling test questions and exams should also be eliminated. Testing firms need to take exam security more seriously, or their exam scores will not be taken less seriously.