If this is really an important thing then Trump has been 'saying' sort of, but doing nothing! The courts don't want every supporter of different candidates swamping them with suits so very few are allowed there. Cruz can't sue himself, but, with Trump #1 and Cruz #2 in the polls, Trump has the best legal standing of anyone to challenge Cruz in court. Trump could do so out of pocket change. Why hasn't he?! Throw it back in Donald's face, Ted!
If I was Trump, I would defer suing an opponent on my own. On the other hand, if the opponent publicly requested a trial, I would oblige. If Ted wants this settled in court, and he wants to be sued to get it there, he need only ask.