My only suggestion is that data_to_hex() looks pretty generically useful,
so I'd put it into dnstest.c, and pass in a static buffer when calling instead
of using isc_mem_allocate().

I pushed that change to the branch, if you're okay with it then I think this
is fine to commit.

(Note, I only evaluated the code, I haven't checked the test case data to
confirm that it tests what it says it does. I take it that's unchanged from
previous versions though.)