There may be such a thing as pure test-taking skill, but I think it boils down to two things: General knowledge/literacy, and knowing the specific format of the test. A well-written test should not have question context for one thing that informs the answer to another question, or have logical inconsistencies/redundancies/ambiguity that can be exploited. Questions should also not build off of each other in a way that causes cascading failure if you can't answer one. These things may happen in low-stakes classroom exams but they certainly shouldn't in a standardized test.

Practically everyone goes to school in our society, so they have very similar opportunity to learn how to take a test. Some of them never get good at it after many years. Maybe that is a sign that they also wouldn't be good at any academic subjects. You wouldn't have a reliable way to measure that, because test-taking is out. I guess they could try to practice test-taking by taking a ton of random tests and grading themselves, but then they might accidentally become experts in some subjects!