Fixes template build failed in release mode (#25751)

Summary:
Fixes https://github.com/facebook/react-native/issues/25745, Xcode stripped dead code in Release mode, and in template test code, it should only run in Debug mode, so we can use a macro to fix this issue.

## Changelog

[iOS] [Fixed] - Fixes template build failed in release mode
Pull Request resolved: https://github.com/facebook/react-native/pull/25751

Test Plan:
1. Create a new project using `react-native init AwesomProject`.
2. Change project target scheme to `Release`.
3. Build and it should success.

Differential Revision: D16442643

Pulled By: TheSavior

fbshipit-source-id: f08ed70523aa1aa418064465f8df367a06e8974f

template/ios/HelloWorldTests/HelloWorldTests.m

@@ -40,11 +40,13 @@
   BOOL foundElement = NO;
 
   __block NSString *redboxError = nil;
+#ifdef DEBUG
   RCTSetLogFunction(^(RCTLogLevel level, RCTLogSource source, NSString *fileName, NSNumber *lineNumber, NSString *message) {
     if (level >= RCTLogLevelError) {
       redboxError = message;
     }
   });
+#endif
 
   while ([date timeIntervalSinceNow] > 0 && !foundElement && !redboxError) {
     [[NSRunLoop mainRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate dateWithTimeIntervalSinceNow:0.1]];
@@ -57,8 +59,10 @@
       return NO;
     }];
   }
-
+  
+#ifdef DEBUG
   RCTSetLogFunction(RCTDefaultLogFunction);
+#endif
 
   XCTAssertNil(redboxError, @"RedBox error: %@", redboxError);
   XCTAssertTrue(foundElement, @"Couldn't find element with text '%@' in %d seconds", TEXT_TO_LOOK_FOR, TIMEOUT_SECONDS);