Is it possible to delegate a domain to IP addresses of DNS servers and not to their names?

No. When a domain name is delegated it shall be necessary to indicate DNS server names.

However, if it is necessary to delegate test.ru domain name with DNS servers which have names in test.ru zone, list of DNS servers shall be indicated as follows:
ns1.test.ru 192.168.15.86
ns2.test.ru 192.168.15.87