Extend Wendelin tutorial notebook script tests
Following the discussion at 111919f5, this change extends the Selenium tests in bt5/erp5_wendelin_tutorial_ui_test by testing that the graphs produced by the notebook scripts contain the correct data.
Since the data points themselves are drawn in a canvas, which is difficult to test in Selenium, we instead test for the presence of correct graph axis labels: since the data used comes from a fixed sample set and the scripts autoscale the axes, we can reasonably assume that the data is correctly plotted if the labels match.
While this isn't perfect, this does effectively break if the data points do not appear.