Marked failing test XFAIL for TestRegisterVariables.test_with_dsym_and_run_command on Darwin

See http://llvm.org/bugs/show_bug.cgi?id=20266


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@212648 91177308-0d34-0410-b5e6-96231b3b80d8