Method CoutStream

unittests/catch.hpp:9453–9453

Store the streambuf from cout up-front because cout may get redirected when running tests

            // Store the streambuf from cout up-front because
            // cout may get redirected when running tests
            CoutStream() : m_os( Catch::cout().rdbuf() ) {}
            ~CoutStream() override = default;

        public: // IStream