Enabled TestBreakAfterJoin.py on Linux.

This is related to:
http://llvm.org/bugs/show_bug.cgi?id=16170

This test ran successfully 40 out of 40 times on Ubuntu 12.04 LTS
x86_64.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@202508 91177308-0d34-0410-b5e6-96231b3b80d8