fix intermittent resolver test error

the resolver test checks that the correct number of fetches have been sent NS rrsets of a given size, but it formerly did so by counting queries received by the authoritative server, which could result in an off-by-one count if one of the queries had been resent due to a timeout or a port number collision.

this commit changes the test to count fetches initiated by the resolver, which should prevent the intermittent test failure, and is the actual datum we were interested in anyway.

Closes #3013 (closed)