cleanup: doh.powerdns.org is not working anymore (#924)

While there, `.../internal/sessionresolver` => `.../sessionresolver`

See https://github.com/ooni/probe/issues/2255

internal/engine/sessionresolver/integration_test.go

@@ -0,0 +1,32 @@
+package sessionresolver_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/sessionresolver"
+	"github.com/ooni/probe-cli/v3/internal/kvstore"
+)
+
+func TestSessionResolverGood(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skip test in short mode")
+	}
+	reso := &sessionresolver.Resolver{
+		KVStore: &kvstore.Memory{},
+	}
+	defer reso.CloseIdleConnections()
+	if reso.Network() != "sessionresolver" {
+		t.Fatal("unexpected Network")
+	}
+	if reso.Address() != "" {
+		t.Fatal("unexpected Address")
+	}
+	addrs, err := reso.LookupHost(context.Background(), "google.com")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(addrs) < 1 {
+		t.Fatal("expected some addrs here")
+	}
+}