Partitioned Search: add a test to check if exception on partition child thread propogates to solver thread and stops all partition child threads

I've tested it manually locally (it works on my machine, but there is no regression test). Follows up on PLANNER-491. Currently the work for that is still on my branch (work in progress): https://github.com/droolsjbpm/optaplanner/pull/229

This isn't easy to test, but some idea's:

• Use a "Solver<TestdataSolution> solver" (there are some unit tests that do this already) by using PlannerTestUtils.buildSolverFactory(...)
• Use a custom SimpleScoreDirector that is designed to throw an exception on 1 (and only 1) partition child thread.
• Check if it bubbles up on the solver thread (it will be in the exception chain).
• Somehow check if all the child threads stopped running... Not sure how to do this properly (so without Thread.sleep()). If you do end up using sleeps, don't use 1 big sleep (for example 10 seconds), but use a loop to do multiple small sleeps that check the condition (for example for (i = 0 -> 50) { sleep(200ms); if (condition) break;}

  , so a successful run takes less than 5 seconds on modern hardware.