When you use WCF test client, you are not using the Discovery feature. It's a classic call to the wsdl/metadata of the service.
Your target Service should be discoverable, by adding something in the host or in the config.
Code
ServiceHost host = new ServiceHost(...);
host.AddServiceEndpoint(new UdpDiscoveryEndpoint());
ServiceDiscoveryBehavior discovery = new ServiceDiscoveryBehavior();
host.Description.Behaviors.Add(discovery);
host.Open();
Config
<services>
<service name = "MyService">
<endpoint
kind = "udpDiscoveryEndpoint"
/>
...
</service>
</services>
<behaviors>
<serviceBehaviors>
<behavior>
<serviceDiscovery/>
</behavior>
</serviceBehaviors>
</behaviors>
At first, also remove the scope in the find criteria.