test-network: set DNS= and NTP= for DHCP server

Now, RoutesToDNS= and RoutesToNTP= are enabled by default on DHCPv4
client. So, if DHCP server picks up DNS or NTP servers from uplink,
then the routes may break CI environment.

Hopefully fixes #19463.

test/test-network/conf/agent-server.network

@@ -4,7 +4,10 @@ Name=server
 Address=192.168.5.1/24
 IPForward=ipv4
 DHCPServer=yes
+
 [DHCPServer]
 BindToInterface=no
 PoolOffset=150
 PoolSize=1
+DNS=192.168.5.1
+NTP=192.168.5.1

test/test-network/conf/dhcp-server-timezone-router.network

@@ -11,3 +11,5 @@ PoolOffset=10
 PoolSize=50
 EmitRouter=yes
 Timezone=Europe/Berlin
+DNS=192.168.5.1
+NTP=192.168.5.1

test/test-network/conf/dhcp-server-with-ipv6-prefix.network

@@ -9,3 +9,7 @@ IPv6SendRA=yes
 
 [IPv6Prefix]
 Prefix=2002:da8:1:0::/64
+
+[DHCPServer]
+DNS=192.168.5.1
+NTP=192.168.5.1