Fix flaky trace dir create test (facebookincubator#10874)

Summary:
Lock is required for multithreading update the created trace directory, verified the fix
in Meta test infra

Pull Request resolved: facebookincubator#10874

Reviewed By: spershin

Differential Revision: D61897242

Pulled By: xiaoxmeng

fbshipit-source-id: 1662388e256d7add8abc05de6faf7b6a00c8f972

velox/exec/trace/test/QueryTraceTest.cpp

@@ -337,19 +337,21 @@ TEST_F(QueryTracerTest, traceDir) {
   fs->mkdir(parentDir);
 
   constexpr auto numThreads = 5;
-  std::vector<std::thread> queryThreads;
-  queryThreads.reserve(numThreads);
+  std::vector<std::thread> traceThreads;
+  traceThreads.reserve(numThreads);
+  std::mutex mutex;
   std::set<std::string> expectedDirs;
   for (int i = 0; i < numThreads; ++i) {
-    queryThreads.emplace_back([&, i]() {
+    traceThreads.emplace_back([&, i]() {
       const auto dir = fmt::format("{}/s{}", parentDir, i);
       trace::createTraceDirectory(dir);
+      std::lock_guard<std::mutex> l(mutex);
       expectedDirs.insert(dir);
     });
   }
 
-  for (auto& queryThread : queryThreads) {
-    queryThread.join();
+  for (auto& traceThread : traceThreads) {
+    traceThread.join();
   }
 
   const auto actualDirs = fs->list(parentDir);