If you have have a method you want to test, but it does some stuff you like for convenience, and you don’t really want to mess for the sake of testing, create with some overrides and do your testing there.

+ (void)foo:(NSURLSessionConfiguration *)sessionConfiguration
    TokenManager *tokenManager = [[TokenManager alloc] init];
    AccessToken *accessToken = tokenManager.accessToken;

    [self addAuthorizationHeaderToSessionConfiguruation:sessionConfiguration token:accessToken];

+ (void)foo:(NSURLSessionConfiguration *)sessionConfiguration token:(AccessToken *)accessToken
    NSString *accessTokenHeaderValue = [NSString stringWithFormat:@"%@ %@", accessToken.tokenType, accessToken.token];
    sessionConfiguration.HTTPAdditionalHeaders = @{@"Authorization" : accessTokenHeaderValue};

The first method is the convenient public one.
The second is the one you can test and inject other things into.