add: Loopback connection client for testing purposes

`get_send_data()` doesn't work yet as I haven't found a C++ style way to safely cast const std::string& to std::string in line 24 of loopback_connection.cpp

src/test/loopback_connection_client.cpp

+/*
+ * loopback_connection_client.cpp
+ *
+ *  Created on: 05.01.2021
+ *      Author: doralitze
+ */
+
+#include "test/loopback_connection_client.hpp"
+
+#include <string.h>
+
+namespace rmrf::test {
+	loopback_connection_client::loopback_connection_client(rmrf::net::connection_client::incomming_data_cb mut_send_data_cb_) :
+			rmrf::net::connection_client{}, mut_send_data_cb(mut_send_data_cb_), send_data_archive{} {
+		// Does nothing special
+	}
+
+	loopback_connection_client::~loopback_connection_client() {
+		// Also doesn't do anything fancy.
+	}
+
+	void loopback_connection_client::write_data(const std::string& data) {
+		// TODO fixme
+		//this->send_data_archive.push_back(data_ptr);
+		if(this->mut_send_data_cb != nullptr) {
+			this->mut_send_data_cb(data);
+		}
+	}
+
+	void loopback_connection_client::send_data_to_incomming_data_cb(const std::string& data) {
+		if(this->in_data_cb != nullptr) {
+			this->in_data_cb(data);
+		}
+	}
+
+	std::vector<std::string> loopback_connection_client::get_send_data() {
+		return this->send_data_archive;
+	}
+
+}
+
+

src/test/loopback_connection_client.hpp

+/*
+ * loopback_connection_client.hpp
+ *
+ *  Created on: 05.01.2021
+ *      Author: doralitze
+ */
+
+#pragma once
+
+#include <vector>
+
+#include "net/connection_client.hpp"
+
+namespace rmrf::test {
+
+/**
+ * Use this cass to mock a connection client.
+ */
+class loopback_connection_client : public rmrf::net::connection_client {
+private:
+	const rmrf::net::connection_client::incomming_data_cb mut_send_data_cb;
+	const std::vector<std::string> send_data_archive;
+public:
+	/**
+	 * This constructor uses the given callback to notify the test suite that the module under test
+	 * send data.
+	 */
+	loopback_connection_client(rmrf::net::connection_client::incomming_data_cb mut_send_data_cb_);
+
+	/**
+	 * Just the mandatory virtual destructor.
+	 */
+	virtual ~loopback_connection_client();
+
+	/**
+	 * This method gets called by the module under test as it simulates the behavior of a normal connection client.
+	 * @param data The data the module wants to send.
+	 */
+	virtual void write_data(const std::string& data);
+
+	/**
+	 * This method sends data to the connections incoming data callback.
+	 * Use it to mimic a remote client sending data.
+	 *
+	 * @param data The data to send.
+	 */
+	void send_data_to_incomming_data_cb(const std::string& data);
+
+	/**
+	 * Use this method in order to get all data the module under test has send.
+	 */
+	std::vector<std::string> get_send_data();
+};
+
+}