A settlement has been reached in a lawsuit challenging the $6.1 billion purchase of clinical research firm Covance by Burlington-based medical testing company Laboratory Corp. of America. The agreement comes less than two weeks before the deal is expected to close, with Covance shareholders slated to vote on the acquisition on Feb. 18. A shareholder of Covance initially brought the lawsuit seeking to block the deal based on allegations that the company's board should not have hired Goldman Sachs as its financial adviser because of a conflict of interest.