add splitter tests

test/test_converter.cpp

@@ -1,26 +1,7 @@
+#include "test_helpers.hpp"
 #include <algorithm>
 #include <ss/converter.hpp>
 
-#ifdef CMAKE_GITHUB_CI
-#include <doctest/doctest.h>
-#else
-#include <doctest.h>
-#endif
-
-class buffer {
-    constexpr static auto buff_size = 1024;
-    char data_[buff_size];
-
-public:
-    char* operator()(const char* data) {
-        memset(data_, '\0', buff_size);
-        strcpy(data_, data);
-        return data_;
-    }
-};
-
-static buffer buff;
-
 TEST_CASE("testing split") {
     ss::converter c;
     for (const auto& [s, expected, delim] :