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.
This can cause problems for some KnowledgeBase implementations when reporting statistics since the list of found causes can contains causes that are not in the knoledge base and hence has no key in whatever database it uses.

A QnD way of fixing this could be to not include them in the statistics logging at all, i.e. add them to the list after the stat logging call.