Use trust anchors with 'dnssec-validation yes' in system tests

Explicitly use an empty 'trust-anchors' statement in the system
tests where it was used implicitly before.

In resolver/ns5/named.conf.in use the trust anchor in 'trusted.conf',
which was supposed to be used there.

bin/tests/system/proxy/ns1/named.conf.in

@@ -54,6 +54,8 @@ options {
 	tcp-initial-timeout 1200;
 };
 
+trust-anchors { };
+
 zone "example0" {
 	type primary;
 	file "example.db";

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

@@ -49,6 +49,8 @@ options {
 	tcp-initial-timeout 1200;
 };
 
+trust-anchors { };
+
 zone "example" {
 	type primary;
 	file "example.db";