The first test of Democratic Presidential contenders looms not in Iowa but in a Dec. 15 straw vote at the Florida state Democratic convention. The victory could allow one of the six candidates to break out of the pack--and right now, Arkansas Governor Bill Clinton may have the edge. He has the support of four important county chairmen and of Lieutenant Governor Buddy MacKay. But don't count out Iowa Senator Tom Harkin, who has strong labor backing, or Senator Bob Kerrey of Nebraska, who has picked up key support in the Tampa Bay area. Both are working the Sunshine State hard.