JENKINS-24868: Add the ability for robot plugin graph to show only failed tests

Hi Jussi,
As suggested, here's a PR for adding a checkbox to show only failed tests in the robot trend graph. I've made the minimal changes to avoid merge conflicts (the js function zoomToChanges is questionably named now), and I couldn't see any easy way to add unit tests to the areas changed like the graph generation, but I'm happy to do this if you can point me in the right direction.

Let me know any comments.

Ninian