add tests util classes to capture std cout and cerr

tests/include/test_utils.hpp

@@ -0,0 +1,32 @@
+#ifndef _RBT_TEST_UTILS_H_
+#define _RBT_TEST_UTILS_H_
+
+#include <iostream>
+#include <streambuf>
+#include <sstream>
+
+class CoutRedirect {
+public:
+    inline CoutRedirect(std::streambuf* new_buffer): old(std::cout.rdbuf(new_buffer)) {}
+    inline ~CoutRedirect() {std::cout.rdbuf(old);}
+
+private:
+    std::streambuf* old;
+};
+
+
+class CerrRedirect {
+public:
+    inline CerrRedirect(std::streambuf* new_buffer): old(std::cerr.rdbuf(new_buffer)) {}
+    inline ~CerrRedirect() {std::cerr.rdbuf(old);}
+
+private:
+    std::streambuf* old;
+};
+
+class NullBuffer : public std::streambuf {
+public:
+    inline int overflow(int c) override { return c; }
+};
+
+#endif  // _RBT_TEST_UTILS_H_