Fixed race conditions in thread tests; exposed by UBSan

llvm-svn: 178029
diff --git a/libcxx/test/thread/thread.threads/thread.thread.class/thread.thread.member/join.pass.cpp b/libcxx/test/thread/thread.threads/thread.thread.class/thread.thread.member/join.pass.cpp
index 3b278da..32faef4 100644
--- a/libcxx/test/thread/thread.threads/thread.thread.class/thread.thread.member/join.pass.cpp
+++ b/libcxx/test/thread/thread.threads/thread.thread.class/thread.thread.member/join.pass.cpp
@@ -32,7 +32,7 @@
     void operator()()
     {
         assert(alive_ == 1);
-        assert(n_alive == 1);
+        assert(n_alive >= 1);
         op_run = true;
     }
 };