Fix broken TestMultithreaded on Linux.

The clean line is failing under Ubuntu 12.04/gcc.  It cleans fine
without it on MacOSX.  It doesn't clean right on Linux but at least
now it doesn't fail the test.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@208713 91177308-0d34-0410-b5e6-96231b3b80d8