Method match

tests/catch.hpp:4073–4076

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

                bool match( NSString* str ) const override {
                    return  (str != nil || m_substr == nil ) &&
                            [str rangeOfString:m_substr].location == 0;
                }

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