Have you actually been hired? have you got a clear day from which you are on the payroll? If the training and test etc is part of the hiring process then they can change their minds and they do not have to explain why. If however you are on the payroll and have started, it should be a lot more difficult to fire you. There would be a dismissal process to go through and you should be aware of why you are being dismissed. So really it hinges on whether you are actaully hired or not.