I've read over the rule and I am not convinced that the enemy has to keep testing. If they meant every movement phase, they would have said something along the lines of, "...must pass a toughness test at the end of the movement phase (test every phase)...". As is, there is no sense of plurality in the text of the rule. I can certainly see why the confusion might arise, but interpreting it that way would seem to be an extreme case of rules lawyering.