Since its earliest days, “Jackass” has always demanded a degree of intestinal fortitude — both from its audience and its cast members. Of all of the things to be tugged at in “Jackass: Best and Last,” the one that fans may least expect is their heartstrings. Longtime ringleader Johnny Knoxville tears up not once but twice over the course of the series’ purported swan song, including when asked if this fifth film is truly the last.