Ensure tests use mock root server if configured

These tests have ns1 configured as a mock root server. Make sure it is
used in all config files of those tests, otherwise some queries could
leak to root nameservers.

(cherry picked from commit 088fcf9a61)

bin/tests/system/chain/ns2/named.conf.in

@@ -26,6 +26,11 @@ options {
 	notify yes;
 };
 
+zone . {
+       type hint;
+       file "../../_common/root.hint";
+};
+
 zone "example" {
 	type primary;
 	file "example.db.signed";

bin/tests/system/dnssec/ns4/named4.conf.in

@@ -66,6 +66,11 @@ view auth {
 	recursion no;
 	allow-recursion { none; };
 
+	zone "." {
+		type hint;
+		file "../../_common/root.hint";
+	};
+
 	zone secure.example {
 		type secondary;
 		primaries { 10.53.0.3; };