If the Boston Bruins are going to eventually scratch and claw their way into a postseason spot, it’s going to be on the strength of being able to win hockey games in different ways. But it’s also going to manifest itself with strong, gritty defense and errorless goaltending at the top of the list of team traits on most nights, and the B’s showed both of those by winning a war of wills between two clearly fatigued hockey teams in a 1-0 win over the Pittsburgh Penguins on Sunday afternoon at TD Garden.It is not going to be a lasting piece of art hanging in a hockey museum as the Bruins were outshot 27-18 and went a dreadful 0-for-6 on a day when their power play did not come through for them at all.