Change TestStopHookMechanism to @expectedFailureLinux.

Similar to my previous check-in related to:
http://llvm.org/bugs/show_bug.cgi?id=15037

10 out of 10 runs of this failed.  Mark it XFAIL.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@202507 91177308-0d34-0410-b5e6-96231b3b80d8