Method FileStream

test/expected/catch.hpp:13475–13478

            mutable std::ofstream m_ofs;
        public:
            FileStream( StringRef filename ) {
                m_ofs.open( filename.c_str() );
                CATCH_ENFORCE( !m_ofs.fail(), "Unable to open file: '" << filename << "'" );
            }
            ~FileStream() override = default;
        public: // IStream
            std::ostream& stream() const override {