Marked TestFormatters.py XFAIL on Linux per pr20230.

See http://www.llvm.org/bugs/show_bug.cgi?id=20230 for details.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@212486 91177308-0d34-0410-b5e6-96231b3b80d8