Allow importing failed tests as failure causes

Here is the code to import failed tests as failure causes, as discussed via ticket https://issues.jenkins-ci.org/browse/JENKINS-25115

I split the actual code from the configuration part for clarity. Feel free to squash them together if you prefer.

One note regarding the configuration page: I could not find a way to fit the configuration checkbox+textbox visually in the same way as the existing options. With the current implementation, the checkbox is in the leftmost column with the label on the right instead of in the middle with the label on the left. One could shift the entire block to the right by using f:nested instead of f:block, but still the label would be on the right of the checkbox and the left column would be empty.
But then how would the added causes be taken into account? Adding them to a list that is not used further in the code does not make sense, or am I missing something?
I need these causes to be actually known to Jenkins, such that the Build Monitor plugin will display them. I assume that it's not needed to add them to the statistics, but I guess the action is really needed, no?