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.
Yes I am saying that the result from  findFailedTests should end up in the foundCausesList, but after that list has been sent to the statistics logger. You might need to send a clone of the list to the Statistics Logger though because I think it reads the list in a background thread.
Moving the call to StatisticsLogger up or moving the addition of test causes doesn't matter _(I read the diff wrong before, hence my strange blabbering)_


My previous explanations were messy because I read the diff wrong and thought this was combined with the methods below when it wasn't.