User-3.swift

/// Mock Subclass of ImageDownloader
/// Fetch API will return what you were passed while init.
internal class MockImageDownloader: ImageDownloader {

    typealias MockResponse = (UIImage?, Error?)

    /// Mocked response. Pass this as part of init and this will be the response of fetch API
    let mockedResponse: MockResponse

    init(url: URL, response: MockResponse) {
        mockedResponse = response
        super.init(url: url)
    }

    override func fetch(then completion: @escaping (UIImage?, Error?) -> Void) {
        completion(mockedResponse.0, mockedResponse.1)
    }
}

// Note: You should have created XCTestExpectation and wait for response, since this is an asynchronous API testing
let currentUser = User(name: "Tim Cook")
let imageFetcher = MockImageDownloader(url: URL(string: "https://somerandomapp.mlaz.im/images/GkdhaKGjhgj7yjHo")!, response: (UIImage(named: "timcook.png"), nil))
currentUser.fetchProfileImage(fetcher: imageFetcher) { (downloaded) in
    XCTAssertTrue(downloaded, "Downloaded is expected to be true when image is fetched")
}