Make test back button label deterministic (#36404)

dev/integration_tests/ios_add2app/ios_add2app/DualFlutterViewController.m

@@ -15,6 +15,11 @@
 - (void)viewDidLoad {
   [super viewDidLoad];
   self.title = @"Dual Flutter Views";
+  self.navigationItem.backBarButtonItem = [[UIBarButtonItem alloc]
+                                              initWithTitle:@"Back"
+                                                      style:UIBarButtonItemStylePlain
+                                                     target:nil
+                                                     action:nil];
 
   UIStackView* stackView = [[UIStackView alloc] initWithFrame:self.view.frame];
   stackView.axis = UILayoutConstraintAxisVertical;

dev/integration_tests/ios_add2app/ios_add2app/HybridViewController.m

@@ -36,6 +36,11 @@ static NSString *_kPing = @"ping";
   stackView.layoutMargins = UIEdgeInsetsMake(0, 0, 50, 0);
   stackView.layoutMarginsRelativeArrangement = YES;
   [self.view addSubview:stackView];
+  self.navigationItem.backBarButtonItem = [[UIBarButtonItem alloc]
+                                              initWithTitle:@"Back"
+                                                      style:UIBarButtonItemStylePlain
+                                                     target:nil
+                                                     action:nil];
 
   NativeViewController *nativeViewController =
       [[NativeViewController alloc] initWithDelegate:self];

dev/integration_tests/ios_add2app/ios_add2app/NativeViewController.m

@@ -34,6 +34,11 @@
 
     self.title = @"Native iOS View";
     self.view.backgroundColor = UIColor.lightGrayColor;
+    self.navigationItem.backBarButtonItem = [[UIBarButtonItem alloc]
+                                                initWithTitle:@"Back"
+                                                        style:UIBarButtonItemStylePlain
+                                                       target:nil
+                                                       action:nil];
 
     _incrementLabel = [self addIncrementLabel];
     UIStackView* footer = [self addFooter];