Marked linux TestStopHookMultipleThreads as expected failure.

Related to this bug:
http://llvm.org/bugs/show_bug.cgi?id=15037

Previously this was marked as skipped. I tried running it 10 times in
a row and it failed every time. Switched it to XFAIL.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@202506 91177308-0d34-0410-b5e6-96231b3b80d8