If you take an IQ test repeatedly then your score gets better. It's not because you've gotten more intelligent.
Aren't you describing learning? If I take multiple English tests (where the questions vary although within the same category, so there must be overlap) and score better, haven't I just learned English better? I would not say that intelligence is fixed.
Or, do you mean the tests can be 'gamed' in the sense of learning some type of problem in a rote fashion? For the second condition, then, yes, I would have to agree -- a test can be manipulated and the results can be invalidated by such manipulation. But how would this be any different from other test situations? Even those tests that measure frank physical abilities (how far can you throw a javelin test) can be gamed to some degree with training.