They don't need to testify. There's no criminal case here. The Supreme Court said that if the House didn't want to rush through this process they could have subpoenaed, through the court system, to have witnesses appear. They failed to get the court orders and so therefore the House is the one who rushed the process through. Of the witnesses who testified multiple Witnesses said that no crime a couple of the witnesses were like second or third-hand accounts. Furthermore, President Trump himself, the President of the Ukraine, and the transcript show that there was nothing illegal or impeachable on that tape.
The Democrats have been wanted to impeach Trump even before he took the oath of office. This is a clown show, nothing more. It was a totally partisan clown show, not only that, but three Democrats voted against impeachment, one abstained I think, and another Democrat left his party and joined the Republicans because of this charade. It's a Witch Hunt.