the benchmark is incorrect

The benchmark between char* and Qstring [here](https://github.com/igormironchik/md4qt/blob/main/tests/md_benchmark/README.md) is not measuring the same thing. char* preallocates the memory while Qstring doesn't. And no, even after using 'reserve', it won't be measuring the same thing. char* doesn't append anything while the other two are appending which involves at least 1 branch per function call