fix(DTXReactNativeSupport): JS and Content can load in any order

Previously waitForReactNativeLoadWithCompletionHandler would
add an observer for "RCTJavaScriptDidLoadNotification",
then when notified would add a second observer for
"RCTContentDidAppearNotification", and after getting a notification
for both it would call the completion handler.

However, with the React Native new architecture the loading of
the JS and Content can happen in a different order (content first).
This would cause DetoxSync to get stuck when enabling synchronization.

Instead, we register all observers (JS, content, failure) one after
the other, and we use a race-free C11 atomic to count when we
receive a successful JS or Content notification.
After we receive both (or fail), we call the handler as before.

DetoxSync/DetoxSync/ReactNativeSupport/DTXReactNativeSupport.m

@@ -212,23 +212,44 @@ + (void)waitForReactNativeLoadWithCompletionHandler:(void (^)(void))handler
 {
   NSParameterAssert(handler != nil);
 
-  __block __weak id observer;
-  __block __weak id observer2;
-
-  observer = [[NSNotificationCenter defaultCenter] addObserverForName:@"RCTJavaScriptDidLoadNotification" object:nil queue:nil usingBlock:^(NSNotification * _Nonnull note) {
-    [[NSNotificationCenter defaultCenter] removeObserver:observer];
-    [[NSNotificationCenter defaultCenter] removeObserver:observer2];
+  __block __weak id jsObserver;
+  __block __weak id contentObserver;
+  __block __weak id failObserver;
+
+  // JavascriptDidLoad and ContentDidAppear can happen in any order
+  // When we receive a notification (either of them), we set this to 1 (atomically)
+  // If it was already at 1, then we received both, and so we can call the handler
+  static _Thread_local _Atomic int successfulNotificationsReceived;
+  atomic_store(&successfulNotificationsReceived, 0);
+
+  jsObserver = [[NSNotificationCenter defaultCenter] addObserverForName:@"RCTJavaScriptDidLoadNotification" object:nil queue:nil usingBlock:^(NSNotification * _Nonnull note) {
+    [[NSNotificationCenter defaultCenter] removeObserver:jsObserver];
+
+    // If the flag was already at 1 then we just received the 2nd, so we call the handler
+    int expected = 0;
+    if (!atomic_compare_exchange_strong(&successfulNotificationsReceived, &expected, 1))
+      {
+      [[NSNotificationCenter defaultCenter] removeObserver:failObserver];
+      handler();
+      }
+  }];
 
-    observer = [[NSNotificationCenter defaultCenter] addObserverForName:@"RCTContentDidAppearNotification" object:nil queue:nil usingBlock:^(NSNotification * _Nonnull note) {
-      [[NSNotificationCenter defaultCenter] removeObserver:observer];
+  contentObserver = [[NSNotificationCenter defaultCenter] addObserverForName:@"RCTContentDidAppearNotification" object:nil queue:nil usingBlock:^(NSNotification * _Nonnull note) {
+    [[NSNotificationCenter defaultCenter] removeObserver:contentObserver];
 
+    // If the flag was already at 1 then we just received the 2nd, so we call the handler
+    int expected = 0;
+    if (!atomic_compare_exchange_strong(&successfulNotificationsReceived, &expected, 1))
+      {
+      [[NSNotificationCenter defaultCenter] removeObserver:failObserver];
       handler();
-    }];
+      }
   }];
 
-  observer2 = [[NSNotificationCenter defaultCenter] addObserverForName:@"RCTJavaScriptDidFailToLoadNotification" object:nil queue:nil usingBlock:^(NSNotification * _Nonnull note) {
-    [[NSNotificationCenter defaultCenter] removeObserver:observer];
-    [[NSNotificationCenter defaultCenter] removeObserver:observer2];
+  failObserver = [[NSNotificationCenter defaultCenter] addObserverForName:@"RCTJavaScriptDidFailToLoadNotification" object:nil queue:nil usingBlock:^(NSNotification * _Nonnull note) {
+    [[NSNotificationCenter defaultCenter] removeObserver:jsObserver];
+    [[NSNotificationCenter defaultCenter] removeObserver:contentObserver];
+    [[NSNotificationCenter defaultCenter] removeObserver:failObserver];
 
     handler();
   }];