Improve TestConcurrentEvents.py

- code cleanup, improved reporting when failures take place
- ensure known thread is interrupted by using pthread_kill() instead of kill() in the signal worker thread
- above should avoid llvm.org/pr16567 on Mac OS X (though kill() could still cause threads to pop out of existance temporarily)
- added an additional check that all threads have exited after worker threads are all join()ed
- logged llvm.org/pr16603 for the new Linux bug discovered

llvm-svn: 186124