Hockey players have been saying it for as long as there has been hockey: You can’t just flip a switch and start playing well in the playoffs. Well, the Bruins hope like heck that ancient axiom is dead wrong. Because if they want to stick around for a long time during this NHL postseason, they suddenly need to start playing a whole lot better. Can they do it? Yes, of course they can. It’s possible, and it doesn’t hurt they open the Stanley Cup playoffs against the Toronto Maple Leafs, who are back in the postseason for the first time since 2003-04, and could fall into the just-glad-to-be-here trap. The Leafs are the best matchup for the Bruins of the three that were possible going into last night’s Game 48. But even as the Bruins showed hints of improvement during the past half-dozen games, last night’s regular-season finale against Ottawa was like a final exam — one last chance to show they’re ready for the greatly enhanced physical, mental and emotional challenges of the postseason. That exam? The Bruins flunked it, unable to defeat a Senators club they’d beaten in 13 of their previous 14 meetings, dropping a 4-2 decision that meant they went 2-5-2 in their last nine outings — and 11-11-3 in the 25 games since they opened the season a deceptive 17-3-3. Unable to get their problems straightened out in the final weeks, the Bruins now want to forget about the regular season and hope magic occurs against the Leafs.