Test that unsolicited NS in positive answer cannot overwrite current NS

Test that unsolicited NS in positive answer cannot overwrite current NS

Before the fixes for CVE-2025-40778, an unsolicited in-bailiwick NS
record was accepted from a (spoofed) answer, enabling a single spoofed A
query/response to redirect traffic for a whole delegation.

In short, the attacker tries to spoof at least one answer that has the
following form:

    rcode NOERROR
    flags QR AA
    ;QUESTION
    trigger$RANDOM.victim. IN TXT
    ;ANSWER
    trigger$RANDOM.victim. 3600 IN TXT "spoofed answer with extra NS"
    ;AUTHORITY
    victim. 3600 IN NS ns.attacker.
    ;ADDITIONAL

This attack was originally reported as "test case 1".

Co-authored-by: Michał Kępień <michal@isc.org>