The phrase "How to Test Negative for Stupid and Why Washington Never Will" is the title of a new book by Senator John Kennedy. It appears to be a tongue-in- cheek guidebook with Kennedy's humorous and perceptive takes on political life in Washington, alongside his reasons why Washington will never "test negative for stupid," meaning the political culture there is perpetually problematic or foolish. The book includes stories and critiques based on his experiences in politics and inside the Senate. Regarding the phrase "why Washington never will," this reflects Kennedy's critical view that political dysfunction and foolishness are deeply entrenched and unlikely to change in Washington, D.C.
If the question refers specifically to "testing negative for" something else or a different context, please clarify, and I can look into that as well.