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



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@181819 91177308-0d34-0410-b5e6-96231b3b80d8