Adding C++ tests that drive LLDB from multiple threads

- test_breakpoint_callback -- filed llvm.org/pr-16000
- test_listener_resume -- resume a process from a thread waiting on SBListener
- test_listener_event_description -- SBEvent description from SBListener thread
- test_listener_event_process -- query process/thread/stack info from SBListener thread

llvm-svn: 181819