Create a ManagedObjectContext for testing

- (void)setUp
{
    [super setUp];
    NSManagedObjectModel *managedObjectModel = [NSManagedObjectModel mergedModelFromBundles:[NSArray arrayWithObject:[NSBundle mainBundle]]];
    NSPersistentStoreCoordinator *storeCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:managedObjectModel];
    XCTAssertTrue([storeCoordinator addPersistentStoreWithType:NSInMemoryStoreType configuration:nil URL:nil options:nil error:NULL] ? YES : NO, @"Should be able to add in-memory store");
    _managedObjectContext = [[NSManagedObjectContext alloc] init];
    _managedObjectContext.persistentStoreCoordinator = storeCoordinator;
}