refactor(netxlite): make sure we always use netxmocks (#399)

* refactor(netxlite): make sure we always use netmocks

While there, improve logging and make sure we test 100% of the
package with unit tests only. (We don't need to have integration
testing in this package because it's fairly simple/obvious.)

Part of https://github.com/ooni/probe/issues/1505

* further improve logs

internal/netxmocks/resolver.go

@@ -16,17 +16,17 @@ type Resolver struct {
 	MockAddress    func() string
 }
 
-// LookupHost implements Resolver.LookupHost.
+// LookupHost calls MockLookupHost.
 func (r *Resolver) LookupHost(ctx context.Context, domain string) ([]string, error) {
 	return r.MockLookupHost(ctx, domain)
 }
 
-// Address implements Resolver.Address.
+// Address calls MockAddress.
 func (r *Resolver) Address() string {
 	return r.MockAddress()
 }
 
-// Network implements Resolver.Network.
+// Network calls MockNetwork.
 func (r *Resolver) Network() string {
 	return r.MockNetwork()
 }