No, he isn't. Trump's bluff is to put up or shut up. Cruz "calling Trump's bluff" would be to come up with a coherent, scholarly argument explaining why he's eligible. Why doesn't he?
FWIW, his lawyers have produced his argument. It's available as part of the IL election board decision package.
Trump originally brought up the birther issue as he said in one of the debates because Cruz was doing too well in the polls. More recently, Trump has threatened a lawsuit...more churning of doubt to maintain his poll numbers.
Cruz has explained publicly his legal view of his own citizenship. The onus is now on Trump to follow through with his threat of lawsuit. If he does not, then he is just a blowhard pussy, a cowardly lion, all talk and no action just like all those “politicians” he rails about!!!