Cottam Approach,
I guess it depends on your definition...
I would venture away from considering it a test as it is not as if there is a right or wrong answer to each question, and it is not as if there is a pass mark that you are trying to reach by answering the questions "correctly". Instead the way these kind of questionnaires tend to work is that an analysis of your answers allows the company (or whoever you are sitting the questionnaire for) to develop a profile of the kind of person you are, particularly with reference to; your prefered methods of work, how you relate to other people, how you handle stressful or challenging situations etc.
I have a friend who carries out these kind of assessments professionally and it is quite astonishing how accurately they can profile you and understand your behaviours and what makes you tick, even down to issues that seemingly have nothing to do with the questions you were asked to form that profile.
I would generally argue that the questionnaire itself is not something which you can "fail". Its a tool the company use to aid them in their selection process. Depending on the picture of you that this tool gives them they may decide that you are unsuitable, thus you might fail selection as an outcome of the questionnaire, but this is not the same as failing the actual questionnaire itself. All possible results of the questionnaire are entirely valid, although if I had to hazard a guess I'd imagine that there may be a statistical basis for thinking that candidates fitting certain types of profile are less likely to make successful ATCOs and thus NATS may disqualify certain candidates on this basis. I would still suggest that this isn't so much an issue of "failure" as it is of unsuitability for the role or the demands of the job. "Failure" suggests that a candidate is not good enough in reference to certain skills or aptitudes (or something to that effect), where as a candidate may be perfectly good enough and possessive of all the necessary skills and abilities yet unsuitable for a whole range of other reasons (potential to be adversely effected by stress, potentially unlikely to put in the necessary degree of work, personal issues, health issues.... to name but a few possibilities).
I would of course entirely agree with your point that anyone who is successful through dishonesty at this stage is likely doing themselves a disservice and may well be found wanting (with potentially serious consequences) at a later stage.