I am quite cognisant of the difference but often testing flights can become training flights - a struggling student who can't make the grade in some area or is using completely the wrong technique can be quickly rebriefed - whether or not you decide he passes that trip is a broad area for discretion, examination of his previous records, perhaps a chat with his primary/previous instructor.
Equally a training flight can become an assessment of ability/potential which may include the introduction of unbriefed elements.
Maybe the difference between the two HAS to be so rigidly defined in civil training and certainly at ab initio level I would agree entirely. But when you are dealing with experienced pilots then the rigidly enforced system can be relaxed.
If all I had done was test/train to the minimum requirement on SAR, few of those pilots woould have reached their full potential (safely) or increased their professional skill levels.
First and foremost you are creating a relationship between yourself and the student and the instructor has to be flexible in his approach and manner to get the best from the student and let the student gain the most from the sortie.