There is no such thing like an Agile Tester when we assume this to be a person. To me, agile means fast feedback and early as well as frequent adaptation to whatever needs to be adapted to. The instance of a tester that would consume whatever a developer has programmed only manifests the old fashioned way of waterfall like setups with late feedback and little adaptation. And it doesn’t really matter whether the tester would be part of an agile team or not. As long as he is supposed to test after the implementation work has been done he could as well be half a world away. Well, communication would be a little easier when he sits right next to the coders.
The major shift the agile tester has to go through would be the change from being a job title to becoming a role one could take on for some time up to its extreme in TDD where it would become a hat which has to be switched often with other hats. In times of XP and TDD and all that stuff, we would talk about agile testing instead. Agile testing would be any activity from TDD to ATDD or whatever testing happens while implementing.