[ReactNative] Prevent RCTDevLoadingView from creating UIWindow on unit tests

Summary:
`RCTDevLoadingView` was being created from a constructor function and creating
a `UIWindow` when the bridge started loading. The `UIWindow` was crashing on
the unit tests since there's no host app.

React/Base/RCTDevLoadingView.m

@@ -12,6 +12,7 @@
 #import "RCTBridge.h"
 #import "RCTDevLoadingView.h"
 #import "RCTDefines.h"
+#import "RCTUtils.h"
 
 #if RCT_DEV
 
@@ -40,7 +41,7 @@ static void RCTDevLoadingViewSetup()
 
 - (instancetype)init
 {
-  if (self = [super init]) {
+  if ((self = [super init]) && !RCTRunningInTestEnvironment()) {
     [[NSNotificationCenter defaultCenter] addObserver:self
                                              selector:@selector(willStartLoading:)
                                                  name:RCTJavaScriptWillStartLoadingNotification