Ok, I will update my comment to an answer.
First, the error message is very clear. "Illegal static declaration" just means, that the static keyword is placed wrong. Remove it!
As you are trying to mock a static method, you might have believed that you must put the static keyword also. But the documentation for the Mock annotation says:
Method modifiers (including public, final, and even static), however, don't have to be the same.
That simply means, you can mock static methods even without declaring it static.
Hmm ... I strongly feel, that the documentation's wording is a bit confusing. Obviously, it is not an option, but you must not declare it static.