WASHINGTON — Michael T. Flynn, the former national security adviser, backed off his planned testimony in a federal case against a former associate, according to court documents unsealed on Tuesday. Mr. Flynn had previously admitted that he lied on foreign lobbying disclosure forms submitted to the Justice Department but now is blaming his former lawyers, accusing them of filing inaccurate forms without his knowledge. He did not dispute that the filing itself contained false information. His latest gambit could provoke another dramatic and risky confrontation with the federal judge who delayed sentencing Mr. Flynn last year in a separate case...