Rewriting TestMultithreaded.py to solve flakyness on Linux

Summary:
test_listener_event_process_state checks for Threads
and Frames in the multithreaded_queue. The listener_func has
more computational load, which may be latter executed than the
pop leading to the failure. This patch tries to only check for
frames in listener_func as presence of frames also confirms 
prescence of threads and avoids the second push into the 
multithreaded_queue.

Reviewers: lldb-commits, clayborg, labath

Differential Revision: http://reviews.llvm.org/D20091

llvm-svn: 269168