add a system test for AXFR fallback when max-ixfr-ratio is exceeded

also cleaned up the ixfr system test:

- use retry_quiet when applicable
- use scripts to generate test zones
- improve consistency

bin/tests/system/ixfr/ns3/named.conf.in

@@ -37,6 +37,7 @@ view "primary" {
 	zone "test" IN {
 		type primary;
 		file "mytest.db";
+		max-ixfr-ratio 75%;
 	};
 	zone "sub.test" IN {
 		type primary;