Add decorator for intermittently failing test on FreeBSD

This test has intermittently failed on FreeBSD for quite some time when
run as part of the full test suite.  It generally passes when run by
itself.  Mark as expected failure for now to reduce buildbot noise.

llvm.org/pr15039 test fails intermittently on FreeBSD


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@222134 91177308-0d34-0410-b5e6-96231b3b80d8