[Coll] Implement get host address in libxgboost. (#9644)

- Port `xgboost.tracker.get_host_ip` in C++.

tests/cpp/collective/net_test.h

@@ -0,0 +1,41 @@
+/**
+ * Copyright 2022-2023, XGBoost Contributors
+ */
+#pragma once
+
+#include <gtest/gtest.h>
+#include <xgboost/collective/socket.h>
+
+#include <fstream>  // ifstream
+
+#include "../helpers.h"  // for FileExists
+
+namespace xgboost::collective {
+class SocketTest : public ::testing::Test {
+ protected:
+  std::string skip_msg_{"Skipping IPv6 test"};
+
+  bool SkipTest() {
+    std::string path{"/sys/module/ipv6/parameters/disable"};
+    if (FileExists(path)) {
+      std::ifstream fin(path);
+      if (!fin) {
+        return true;
+      }
+      std::string s_value;
+      fin >> s_value;
+      auto value = std::stoi(s_value);
+      if (value != 0) {
+        return true;
+      }
+    } else {
+      return true;
+    }
+    return false;
+  }
+
+ protected:
+  void SetUp() override { system::SocketStartup(); }
+  void TearDown() override { system::SocketFinalize(); }
+};
+}  // namespace xgboost::collective