feat(netxmocks): implement mocks for netxlite.Resolver (#398)

While there, make sure we require using &netxmocks.Dialer.

Still part of https://github.com/ooni/probe/issues/1505

internal/netxmocks/resolver.go

@@ -0,0 +1,34 @@
+package netxmocks
+
+import "context"
+
+// resolver is the interface we expect from a resolver
+type resolver interface {
+	LookupHost(ctx context.Context, domain string) ([]string, error)
+	Network() string
+	Address() string
+}
+
+// Resolver is a mockable Resolver.
+type Resolver struct {
+	MockLookupHost func(ctx context.Context, domain string) ([]string, error)
+	MockNetwork    func() string
+	MockAddress    func() string
+}
+
+// LookupHost implements Resolver.LookupHost.
+func (r *Resolver) LookupHost(ctx context.Context, domain string) ([]string, error) {
+	return r.MockLookupHost(ctx, domain)
+}
+
+// Address implements Resolver.Address.
+func (r *Resolver) Address() string {
+	return r.MockAddress()
+}
+
+// Network implements Resolver.Network.
+func (r *Resolver) Network() string {
+	return r.MockNetwork()
+}
+
+var _ resolver = &Resolver{}