SPARK-2416: Allow richer reporting of unit test results

The built-in Jenkins integration is pretty bad. It's very confusing to users whether tests have passed or failed and we can't easily customize the message.

With some small scripting around the Github API we can do much better than this.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #1340 from pwendell/better-qa-messages and squashes the following commits:

fd6077d [Patrick Wendell] Better automation for unit tests.
Revert "[HOTFIX] Synchronize on SQLContext.settings in tests."

This reverts commit d4c30cd9918e18dde2a52909e36eaef6eb5996ab.
HOTFIX: Remove persistently failing test in master.

Apparently this functionality is going to be removed soon anywyas.