This PR allows SparkQA (i.e. Jenkins) to report in its posts to GitHub what category of test failed, if one can be determined.
The failure categories are:
* general failure
* RAT checks failed
* Scala style checks failed
* Python style checks failed
* Build failed
* Spark unit tests failed
* PySpark unit tests failed
* MiMa checks failed
This PR also fixes the diffing logic used to determine if a patch introduces new classes.
Author: Nicholas Chammas <nicholas.chammas@gmail.com>
Closes#2606 from nchammas/report-failed-test-category and squashes the following commits:
d67df03 [Nicholas Chammas] report what test category failed
