Add decorator for FreeBSD failing test due to pr18061

The test code locks mutexes from one thread and unlocks them from a
different one, which is not valid (it's either undefined behaviour or is
required to return an error).

See http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_lock.html

llvm-svn: 195707

lldb/test/functionalities/thread/TestNumThreads.py

@@ -19,7 +19,7 @@ class NumberOfThreadsTestCase(TestBase):
         self.buildDsym()
         self.number_of_threads_test()
 
-    @expectedFailureFreeBSD("llvm.org/pr16696") # threaded inferior not yet implemented on FreeBSD
+    @expectedFailureFreeBSD("llvm.org/pr18061") # test relies on undefined behaviour
     @dwarf_test
     def test_with_dwarf(self):
         """Test number of threads."""