Sample code to demonstrate URLSession subclassing and injecting to the SUT

SubClass_URLSession.swift

// MARK: - Main Target
final class HTTPClient {
    let session: URLSession

    init(session: URLSession = URLSession.shared) {
        self.session = session
    }

    func get(from url: URL) {
        session.dataTask(with: URLRequest(url: url)).resume()
    }

}

// MARK: - Test Target
final class HTTPClientTests {

    func test_initHTTPClient_sessionNotNil() {
        let sut = makeSUT()
        XCTAssertNotNil(sut.session)
    }

    func makeSUT() -> HTTPClient {
        return HTTPClient(session: URLSessionSpy())
    }
}

final class URLSessionSpy: URLSession {

    override func dataTask(with request: URLRequest) -> URLSessionDataTask {
        // add required logic for capturing data
        return SampleURLSessionDataTask()
    }

    private class SampleURLSessionDataTask: URLSessionDataTask {}
}