fix: Stop embedded tests from hanging on ARM by using atomic_flag (#6248)

This change replaces the mutex `stoppingMutex_`, the `atomic_bool` variable `isTimeToStop`, and the conditional variable `stoppingCondition_` with an `atomic_flag` variable.

When `xrpld` is running the embedded tests as a child process, it has a control thread (the app bundle thread) that starts the application, and an application thread (the thread that executes `app_->run()`). Due to the relaxed memory ordering on ARM, it's not guaranteed that the application thread can see the change of the value resulting from the `isTimeToStop.exchange(true)` call before it is notified by `stoppingCondition_.notify_all()`, even though they do happen in the right order in the app bundle thread in `ApplicationImp::signalStop`. We therefore often get into the situation where `isTimeToStop` is `true`, but the application thread is waiting for `stoppingCondition_` to notify, because the app bundle thread may have already notified before the application thread actually starts waiting.

Switching to a single `atomic_flag` variable makes sure that there's only one synchronisation object and then the memory order guarantee provided by c++ can make sure that `notify_all` gets synchronised after `test_and_set` does.

Fixing this issue will stop the unit tests hanging forever and then we should see less (or hopefully no) time out errors in daily github action runs

(cherry picked from commit bb529d03175fc988a9d7b593bf60e21b6803cbb7)

src/xrpld/app/main/Application.cpp

@@ -228,11 +228,7 @@ public:
 
     boost::asio::signal_set m_signals;
 
-    // Once we get C++20, we could use `std::atomic_flag` for `isTimeToStop`
-    // and eliminate the need for the condition variable and the mutex.
-    std::condition_variable stoppingCondition_;
-    mutable std::mutex stoppingMutex_;
-    std::atomic<bool> isTimeToStop = false;
+    std::atomic_flag isTimeToStop;
 
     std::atomic<bool> checkSigs_;
 
@@ -1585,10 +1581,7 @@ ApplicationImp::run()
         getLoadManager().activateDeadlockDetector();
     }
 
-    {
-        std::unique_lock<std::mutex> lk{stoppingMutex_};
-        stoppingCondition_.wait(lk, [this] { return isTimeToStop.load(); });
-    }
+    isTimeToStop.wait(false, std::memory_order_relaxed);
 
     JLOG(m_journal.debug()) << "Application stopping";
 
@@ -1675,14 +1668,14 @@ ApplicationImp::run()
 void
 ApplicationImp::signalStop(std::string msg)
 {
-    if (!isTimeToStop.exchange(true))
+    if (!isTimeToStop.test_and_set(std::memory_order_acquire))
     {
         if (msg.empty())
             JLOG(m_journal.warn()) << "Server stopping";
         else
             JLOG(m_journal.warn()) << "Server stopping: " << msg;
 
-        stoppingCondition_.notify_all();
+        isTimeToStop.notify_all();
     }
 }
 
@@ -1701,7 +1694,7 @@ ApplicationImp::checkSigs(bool check)
 bool
 ApplicationImp::isStopping() const
 {
-    return isTimeToStop.load();
+    return isTimeToStop.test(std::memory_order_relaxed);
 }
 
 int