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.
The reason I added static is because this is used from the static 'scan' method in BuildFailureScanner and this would throw a compilation error. How to handle it correctly?