attempted test fixes

tests/chapter2-exercise-scrollbar-tests.md

@@ -38,6 +38,7 @@ Resize the window and set up the URL and web page:
     >>> wbemocks.socket.respond_200(url, content)
     >>> browser.set_parameters(WIDTH=800, HEIGHT=100,
     ...     VSTEP=20, HSTEP=800, SCROLL_STEP=25)
+    >>> wbemocks.MockCanvas.hide_above(-20)
 
 Since the h-step is equal to the width and the page contents is 10
 letters, there should be 10 lines of text. Since each line is 20