Fix FallbackTooManyRecordsAxfrHandler to follow convention

All the other subclasses AxfrHandler send three messages.
This oversight was inherited from the original Perl implementation of
the server and was not fixed in 46ecbbe where it was rewritten.

This allows refactoring and sharing of the superclass.

(cherry picked from commit 93a8079489)

bin/tests/system/ixfr/ans2/ans.py

@@ -196,8 +196,8 @@ class TooManyRecordsIxfrHandler(IxfrHandler):
 
 class FallbackTooManyRecordsAxfrHandler(AxfrHandler):
     answers = (
+        (soa(3),),
         (
-            soa(3),
             ns(),
             txt("fallback AXFR on too many records"),
         ),