The U.S. Supreme Court is formally sending Donald Trump’s Jan. 6 case back down to a district court — and an expert has some advice for prosecutor Jack Smith on the best way to secure a win. The high court granted Trump broad immunity for the official acts he undertook as president and established a three-part test that District Judge Tanya Chutkan will now apply to determine which crimes alleged by special counsel Smith in his federal election interference case will survive the Supreme Court ruling — a hearing which legal expert Norm Eisen says will resemble a mini-trial. “Writing for a 6-3 m…