They must have been having a good laugh down in Miami on Saturday and not just at what the team in green was doing at the Garden in what has to rank as one of the all-time low points for a very proud Boston Celtics franchise. You could even imagine some members of the Heat watching all the bad basketball being played by the Knicks and saying, “If they’re supposed to be our toughest test in the East, we can book our flights to Oklahoma City.’’ Except for the last five or six minutes of their 2013 playoff opener, the Knicks didn’t look like they belong on the same court with the defending champs, even if they did come away with an 85-78 victory. The Celtics were so feeble, scoring all of eight points in the fourth quarter, they didn’t belong on the same court with the Charlotte Bobcats. Eight points in a quarter on three baskets? Twenty-five points for a half? Larry Bird used to get those totals all by himself. “We couldn’t put the ball in the basket,’’ Kevin Garnett said, in the understatement of the year. That, plus eight turnovers in the final 12 minutes, doomed the…