Fake a neighboring class method for the object in test. Stolen from https://gist.github.com/1038034 with a clearer (to me) interface and example.

ExampleTests.m

#import <SenTestingKit/SenTestingKit.h>
#import "SenTestCase+MethodSwizzling.h"

#pragma mark Fakes for Tests

@interface FakeMyObject : NSObject

/**
 * A fake, or "stub," implementation of MyObject's +doSomething method.
 */
- (id)fakeDoSomething

@end

@implementation FakeMyObject

- (id)fakeDoSomething
{
    NSLog(@"%s", __PRETTY_FUNCTION__);
    id someFakeReturnObj = [[NSObject alloc] init];
    return someFakeReturnObj;
}

@end

#pragma mark Tests

@interface ExampleTests : SenTestCase

@end

@implementation ExampleTests

- (void)testSomeBehavior
{
    [self swizzleClassMethod:@selector(doSomething)
                     inClass:[MyObject class]
                withClassMethod:@selector(fakeDoSomething)
                   fromClass:[FakeMyObject class]
                executeBlock:^
                {
                    id objectInTest = [[Foo alloc] init];
                    [objectInTest someBehavior]; // some action that uses MyObject's
                                                 // doSomething class method
                    STAssertTrue(objectInText.someState,
                                 @"someState should be true due to someBehavior");
                }
    ];
}

@end

SenTestCase+MethodSwizzling.h

#import <SenTestingKit/SenTestingKit.h>

/**
 * Category on top of SenTestCase to provide convenience methods for changing
 * the implementation of some selector (i.e., stubbing) for execution of a block
 * where test assertions can be made.
 *
 * Cribbed from https://gist.github.com/1038034
 */
@interface SenTestCase (MethodSwizzling)

/**
 * Changes the implementation of some class method for the execution of a block.
 *
 * @param aOriginalMethod
 *            The selector of the original method that needs to be faked.
 * @param aOriginalClass
 *            The class containing the original method that needs to be faked.
 * @param aNewMethod
 *            The selector for the fake implementation that will be used within
 *            the test.
 * @param aNewClas
 *            The class containing the fake implementation that will be used
 *            within the test.
 * @param aBlock
 *            The block that executes within the context of the fake
 *            implementation.
 */
- (void)swizzleClassMethod:(SEL)aOriginalMethod
                   inClass:(Class)aOriginalClass
            withClassMethod:(SEL)aNewMethod
                 fromClass:(Class)aNewClass
              executeBlock:(void (^)(void))aBlock;

@end

SenTestCase+MethodSwizzling.m

#import "SenTestCase+MethodSwizzling.h"

#include <objc/runtime.h>

@implementation SenTestCase (MethodSwizzling)

- (void)swizzleClassMethod:(SEL)aOriginalMethod
                   inClass:(Class)aOriginalClass
           withClassMethod:(SEL)aNewMethod
                 fromClass:(Class)aNewClass
              executeBlock:(void (^)(void))aBlock
{
    Method originalMethod = class_getClassMethod(aOriginalClass, aOriginalMethod);
    Method mockMethod = class_getClassMethod(aNewClass, aNewMethod);
    method_exchangeImplementations(originalMethod, mockMethod);
    aBlock();
    method_exchangeImplementations(mockMethod, originalMethod);
}

@end