Re-enable TestExprDoesntBlock.py on Linux.

This is related to:
http://llvm.org/bugs/show_bug.cgi?id=15258

I ran this test 10 times successfully against Ubuntu 12.04 LTS x86_64
with lldb built with gcc 4.8.2 and July 2013 libedit.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@202456 91177308-0d34-0410-b5e6-96231b3b80d8