message: Relax a data formatter test
  Before inspecting the contents of a list, make sure that we've stepped
  past the push_back() that inserts the element we're interested in.
diff: @@ -190,6 +190,7 @@ class LibcxxListDataFormatterTestCase(TestBase):
  self.runCmd("n") # This gets us past the printf
  self.runCmd("n")
  + self.runCmd("n")
  # check access-by-index
  self.expect("frame variable text_list[0]",
  