Add assert for test of plot example

[sverhoeven]

The src/js/example-plot.html is tested in https://github.com/NLESC-JCER/cpp2wasm/blob/master/TESTING.md, but there is no check if the plot has been drawn correctly.

Testing is hard due to vega rendering to canvas and different duration's each run

[fdiblen]

Maybe we can export the chart data and check this in a test.