'assert that lines2 are contained (linearly) in lines1.
return a list of extralines found.'

def assert_contains_lines(self, lines2):

    __tracebackhide__ = True
    val = self.stringio.getvalue()
    self.stringio.truncate(0)
    self.stringio.seek(0)
    lines1 = val.split('\n')
    return LineMatcher(lines1).fnmatch_lines(lines2)
