I think artistic creativity is very different from mechanical or mathematical creativity.
Artistry is about putting things together in a unique way that activates human emotion.
This type of skill is very different from the spatial intelligence or pattern recognition skills that are important for general problem solving.Art is skill and artistic skill can be measured like an IQ test, using tests. How do you think people get into art academies?
As for being a ‘great’ artist or author, the cases behind aren't mysterious qualities like creativity but is partly IQ, partly special skills that can be measured by a test, and luck. The luck here is life experience: interesting life experiences + IQ + skills = art.
If you can't test it, it doesn't exist or is a wider range of causes (i.e., mostly luck).
You can’t test whether or not I am conscious, yet I know for a fact that I am.