By MARTHA DELLER FORT WORTH — Tarrant County prosecutors are expected to rest their case in a capital murder trial today after calling the grandson of a 68-year-old Arlington woman who was stabbed to death during what prosecutors have described as a crime spree by a 25-year-old defendant who also killed his uncle and wounded his own girlfriend.Prosecutors Lloyd Whelchel and Page Simpson called nearly 20 witnesses last week in the trial of Dustin Nall, accused of the 2007 robbery-slaying of Bertha Wilkerson.Wilkerson’s grandson, Spencer Combs, is expected to testify about chasing Nall out of the home with a shotgun as his grandmother lay bleeding in front of the duplex in north central Arlington.A DNA analyst testified Friday that Wilkerson’s blood was on the knife that Nall threw out as he fled from officers in Randol Mill Park minutes after Wilkerson’s slaying.