If run from the wrong directory, std::ifstream f("test/data/big-list-of-naughty-strings/blns.json"); will not find the file and thus f.eof() will never return true.
Use canonical C++ file reading loop from https://gehrcke.de/2011/06/reading-files-in-c-using-ifstream-dealing-correctly-with-badbit-failbit-eofbit-and-perror/ instead.