Fix TestEvents.py on OS X

This change addresses a hang/segfault in TestEvents.py. The threads that
run the listener loops now do an SBListener.Clear() before they wrap up
their work. This prevents the test from trying to clean up the
SBListener too late.

There is a separate issue here which is that we should prevent this
clean-up time lock-up, but that is out of scope for this particular
change. I'd like to get these tests back and running the normal flow
rather than skipping them.

This addresses:
llvm.org/pr25924 (at least, the OS X side, although I suspect this will
also address Linux)

http://reviews.llvm.org/D19983
reviewed by: Jim Ingham

llvm-svn: 268653