[Tests] fix non-determinism failure in testcase

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@349460 91177308-0d34-0410-b5e6-96231b3b80d8

runtime/test/ompt/parallel/nested_threadnum.c

@@ -7,6 +7,7 @@ int main() {
   omp_set_nested(1);
 #pragma omp parallel num_threads(2)
   {
+#pragma omp barrier
 #pragma omp parallel num_threads(2)
     { print_frame(0); }
   }