To prevent application logic from interfering with unit tests.

gistfile1.m

static BOOL isRunningTests(void) __attribute__((const));
 
- (BOOL)application:(UIApplication *)application   
        didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    if (isRunningTests()) {
        return YES;
    }
    
    //
    // Normal logic goes here
    //
    
    return YES;
}
 
static BOOL isRunningTests(void)
{
    NSDictionary* environment = [[NSProcessInfo processInfo] environment];
    NSString* injectBundle = environment[@"XCInjectBundle"];
    NSString* pathExtension = [injectBundle pathExtension];
 
    return ([pathExtension isEqualToString:@"octest"] ||
            [pathExtension isEqualToString:@"xctest"]);
}

This is a modified version of the snippet from this excellent article:
http://www.objc.io/issue-1/testing-view-controllers.html

I added compatibility for XCode5's newer XCTest framework.

This is also mentioned in my blog post:
http://www.rsaunders.co.uk/2014/01/preventing-unit-tests-from-crashing.html