Method match

tests/catch.hpp:4047–4050

                Equals( NSString* substr ) : StringHolder( substr ){}

                bool match( NSString* str ) const override {
                    return  (str != nil || m_substr == nil ) &&
                            [str isEqualToString:m_substr];
                }

                std::string describe() const override {
                    return "equals string: " + Catch::Detail::stringify( m_substr );