Switch out DISABLE_YELLOW_BOX for IS_TESTING

Summary: Switch to using IS_TESTING on the Platform module. While IS_TESTING has to be explicitly set in the test harness, this makes it more usable and stops people from relying on brittle variables in the (larger) environment.

Reviewed By: fkgozali

Differential Revision: D4423661

fbshipit-source-id: 27a80867778b9374bcba67b69f9c93d32c0a74b0

React/Base/RCTPlatform.m

@@ -30,6 +30,11 @@ static NSString *interfaceIdiom(UIUserInterfaceIdiom idiom) {
   }
 }
 
+static BOOL isTestingEnvironment(void) {
+  NSDictionary *environment = [[NSProcessInfo processInfo] environment];
+  return [environment[@"IS_TESTING"] boolValue];
+}
+
 RCT_EXPORT_MODULE(IOSConstants)
 
 - (NSDictionary<NSString *, id> *)constantsToExport
@@ -40,6 +45,7 @@ RCT_EXPORT_MODULE(IOSConstants)
     @"osVersion": [device systemVersion],
     @"systemName": [device systemName],
     @"interfaceIdiom": interfaceIdiom([device userInterfaceIdiom]),
+    @"isTesting": @(isTestingEnvironment()),
   };
 }