gistfile1.txt

Usage:

- (void)testCreateNewDocument
{
    [self mn_preformWithTimeout:10 asynchronousTest:^{
		[[MNDocumentController sharedDocumentController] createNewDocumentInFolder:nil withCompletionHandler:^(MNDocument *document, MNDocumentReference *reference) {
			XCTAssertTrue(document && reference, @"Failed to create document.");
			[self mn_finishRunningAsynchronousTest];
		}];
	}];
}

XCTestCase+MNAsynchronousTestCase.h

//
//  XCTestCase+MNAsynchronousTestCase.h
//  MindNodeTouch
//
//  Created by Markus Müller-Simhofer on 28.07.13.
//  Copyright (c) 2013 IdeasOnCanvas GmbH. All rights reserved.
//

#import <XCTest/XCTest.h>

@interface XCTestCase (MNAsynchronousTestCase)

- (void)mn_preformWithTimeout:(NSTimeInterval)timeout asynchronousTest:(void (^)(void))asynchronousTest;
- (void)mn_finishRunningAsynchronousTest;

@end

XCTestCase+MNAsynchronousTestCase.m

//
//  XCTestCase+MNAsynchronousTestCase.m
//  MindNodeTouch
//
//  Created by Markus Müller-Simhofer on 28.07.13.
//  Copyright (c) 2013 IdeasOnCanvas GmbH. All rights reserved.
//

#import "XCTestCase+MNAsynchronousTestCase.h"
#import <objc/runtime.h>


static char mn_waitingForAsynchronousTestKey;

@implementation XCTestCase (MNAsynchronousTestCase)

- (void)mn_preformWithTimeout:(NSTimeInterval)timeout asynchronousTest:(void (^)(void))asynchronousTest
{
    // run test block
	[self mn_setWaitingForAsynchronousTest:YES];
	asynchronousTest();
	
	NSTimeInterval timeoutTime = [[NSDate dateWithTimeIntervalSinceNow:timeout] timeIntervalSinceReferenceDate];
	while ([self mn_waitingForAsynchronousTest])  {
		[[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:0.1f]];
		if ([NSDate timeIntervalSinceReferenceDate] > timeoutTime) {
			XCTFail(@"Test timed out! Did you forget to call -mn_finishRunningAsynchronousTest");
			[self mn_setWaitingForAsynchronousTest:NO];
		}
	}
}



- (void)mn_finishRunningAsynchronousTest
{
	[self mn_setWaitingForAsynchronousTest:NO];
}


- (void)mn_setWaitingForAsynchronousTest:(BOOL)isWaiting;
{
	objc_setAssociatedObject(self, &mn_waitingForAsynchronousTestKey, @(isWaiting), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}


- (BOOL)mn_waitingForAsynchronousTest
{
	NSNumber *valueObject = objc_getAssociatedObject(self, &mn_waitingForAsynchronousTestKey);
	return [valueObject boolValue];
}

@end

I used to do it without blocks on OCUnit. As far as I understand XCTesting works just the same.
https://gist.github.com/n-b/2299695

I find that using a stop flag makes it a lot simpler, but there is still room for improvement.
For example, the async test block could return a BOOL indicating if the test is completed.