Fixed race conditions in thread tests; exposed by UBSan

llvm-svn: 178029

libcxx/test/thread/thread.threads/thread.thread.class/thread.thread.algorithm/swap.pass.cpp

@@ -32,7 +32,7 @@ public:
     void operator()()
     {
         assert(alive_ == 1);
-        assert(n_alive == 1);
+        assert(n_alive >= 1);
         op_run = true;
     }
 };