'As in Bok Choi, but instead of a CSS selector, we pass a
block_id. We may want to be able to pass an optional selector
for picking a subelement of the block.
This confirms status code, and that the screenshot is
identical.
To do: Implement'

def assertXBlockScreenshot(self, block_urlname, rendering=None):

    raise NotImplementedError("We    need    Ben's    help    to    finish    this")
