tests/dnsd: read config from file

Make the <dns> tag in a test case control what is stored there. Also
documented. Make test 2102 and 2103 use the new tag.

Lets the test case config the A and AAAA contents the server replies
with. Initial work for the HTTPS RR exists, but does not yet work.

Closes #17543

docs/tests/FILEFORMAT.md

@@ -383,6 +383,14 @@ issue.
 `writedelay: [secs]` delay this amount between reply packets (each packet
   being 512 bytes payload)
 
+### `<dns>`
+
+Commands for the test DNS server.
+
+- `A: [dotted ipv4 address]` - set IPv4 address to return
+- `AAAA: [numerical IPv6 address]` - set IPv6 address to return, with or
+  without `[]`
+
 ## `<client>`
 
 ### `<server>`