If you explore the subject of testing enough, you'll eventually come across the advice to avoid static methods and variables. I'll explain more about static variables and let you look into static methods yourself—there are plenty articles readily searchable on the topic.
Static non-final variables present a problem for testing in that they hold globally (to whatever degree their declared visibility allows, that is) accessible data. This is problematic when you need tests to be independent of each other (The "I" in the FIRST acronym for tests: Fast, Independent/Isolated, Repeatable, Self-verifying, and Timely). Global state messes up Independence by potentially making one operation dependent on the result of other operations performed before it. Immutability, of course, eliminates that problem, hence the advice to make static variables final so that their values won't ever change.
The best ideas are the crazy ones. If you have a crazy idea and it works, it's really valuable.—Kent Beck