libstdc++: Fix some more deadlocks in tests [PR 97936]

The missed notifications fixed in r11-5383 also happen in some other
tests which have similar code.

libstdc++-v3/ChangeLog:

	PR libstdc++/97936
	* testsuite/29_atomics/atomic/wait_notify/bool.cc: Fix missed
	notifications by making the new thread wait until the parent
	thread is waiting on the condition variable.
	* testsuite/29_atomics/atomic/wait_notify/pointers.cc: Likewise.
	* testsuite/29_atomics/atomic_flag/wait_notify/1.cc: Likewise.
	* testsuite/29_atomics/atomic_ref/wait_notify.cc: Likewise.

libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/bool.cc

@@ -36,11 +36,16 @@ main ()
 
   std::mutex m;
   std::condition_variable cv;
+  std::unique_lock<std::mutex> l(m);
 
   std::atomic<bool> a(false);
   std::atomic<bool> b(false);
   std::thread t([&]
 		{
+		  {
+		    // This ensures we block until cv.wait(l) starts.
+		    std::lock_guard<std::mutex> ll(m);
+		  }
 		  cv.notify_one();
 		  a.wait(false);
 		  if (a.load())
@@ -48,7 +53,6 @@ main ()
 		      b.store(true);
 		    }
 		});
-  std::unique_lock<std::mutex> l(m);
   cv.wait(l);
   std::this_thread::sleep_for(100ms);
   a.store(true);

libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/pointers.cc

@@ -36,6 +36,7 @@ main ()
 
   std::mutex m;
   std::condition_variable cv;
+  std::unique_lock<std::mutex> l(m);
 
   long aa;
   long bb;
@@ -43,12 +44,15 @@ main ()
   std::atomic<long*> a(nullptr);
   std::thread t([&]
 		{
+		  {
+		    // This ensures we block until cv.wait(l) starts.
+		    std::lock_guard<std::mutex> ll(m);
+		  }
 		  cv.notify_one();
 		  a.wait(nullptr);
 		  if (a.load() == &aa)
 		    a.store(&bb);
 		});
-  std::unique_lock<std::mutex> l(m);
   cv.wait(l);
   std::this_thread::sleep_for(100ms);
   a.store(&aa);

libstdc++-v3/testsuite/29_atomics/atomic_flag/wait_notify/1.cc

@@ -36,18 +36,22 @@ main()
 
   std::mutex m;
   std::condition_variable cv;
+  std::unique_lock<std::mutex> l(m);
 
   std::atomic_flag a;
   std::atomic_flag b;
   std::thread t([&]
 		{
+		  {
+		    // This ensures we block until cv.wait(l) starts.
+		    std::lock_guard<std::mutex> ll(m);
+		  }
 		  cv.notify_one();
 		  a.wait(false);
 		  b.test_and_set();
 		  b.notify_one();
 		});
 
-  std::unique_lock<std::mutex> l(m);
   cv.wait(l);
   std::this_thread::sleep_for(100ms);
   a.test_and_set();

libstdc++-v3/testsuite/29_atomics/atomic_ref/wait_notify.cc

@@ -37,17 +37,21 @@ Tp check_wait_notify(Tp val1, Tp val2)
 
   std::mutex m;
   std::condition_variable cv;
+  std::unique_lock<std::mutex> l(m);
 
   Tp aa = val1;
   std::atomic_ref<Tp> a(aa);
   std::thread t([&]
 		{
+		  {
+		    // This ensures we block until cv.wait(l) starts.
+		    std::lock_guard<std::mutex> ll(m);
+		  }
 		  cv.notify_one();
 		  a.wait(val1);
 		  if (a.load() != val2)
 		    a = val1;
 		});
-  std::unique_lock<std::mutex> l(m);
   cv.wait(l);
   std::this_thread::sleep_for(100ms);
   a.store(val2);