[libc++] Fix sporadic test failure in condition_variable notify_all test (llvm#97622)

fixes llvm#95944

Author: huixie90

libcxx/test/std/thread/thread.condition/thread.condition.condvar/notify_all.pass.cpp

@@ -14,6 +14,7 @@
 
 // void notify_all();
 
+#include <atomic>
 #include <condition_variable>
 #include <mutex>
 #include <thread>
@@ -29,10 +30,13 @@ int test0 = 0;
 int test1 = 0;
 int test2 = 0;
 
+std::atomic<int> ready_count(0);
+
 void f1()
 {
     std::unique_lock<std::mutex> lk(mut);
     assert(test1 == 0);
+    ready_count += 1;
     while (test1 == 0)
         cv.wait(lk);
     assert(test1 == 1);
@@ -43,6 +47,7 @@ void f2()
 {
     std::unique_lock<std::mutex> lk(mut);
     assert(test2 == 0);
+    ready_count += 1;
     while (test2 == 0)
         cv.wait(lk);
     assert(test2 == 1);
@@ -53,7 +58,9 @@ int main(int, char**)
 {
     std::thread t1 = support::make_test_thread(f1);
     std::thread t2 = support::make_test_thread(f2);
-    std::this_thread::sleep_for(std::chrono::milliseconds(100));
+    while (ready_count.load() != 2) {
+      std::this_thread::sleep_for(std::chrono::milliseconds(100));
+    }
     {
         std::unique_lock<std::mutex>lk(mut);
         test1 = 1;