XFAIL a test that fails on FreeBSD 11 (local and buildbot)

For some reason this happens only when running the full test suite
(e.g., via ninja check-lldb), but not when running the
TestStaticVariables.py tests in isolation. XFAIL for now while
investigating, in an attempt to bring the bot to green and reduce noise.

llvm.org/pr20550

llvm-svn: 236993