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/lookup_test.go

@@ -0,0 +1,44 @@
+package sessionresolver
+
+import (
+	"context"
+	"errors"
+	"io"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/ooni/probe-cli/v3/internal/model/mocks"
+)
+
+func TestTimeLimitedLookupSuccess(t *testing.T) {
+	expected := []string{"8.8.8.8", "8.8.4.4"}
+	re := &mocks.Resolver{
+		MockLookupHost: func(ctx context.Context, domain string) ([]string, error) {
+			return expected, nil
+		},
+	}
+	ctx := context.Background()
+	out, err := timeLimitedLookup(ctx, re, "dns.google")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if diff := cmp.Diff(expected, out); diff != "" {
+		t.Fatal(diff)
+	}
+}
+
+func TestTimeLimitedLookupFailure(t *testing.T) {
+	re := &mocks.Resolver{
+		MockLookupHost: func(ctx context.Context, domain string) ([]string, error) {
+			return nil, io.EOF
+		},
+	}
+	ctx := context.Background()
+	out, err := timeLimitedLookup(ctx, re, "dns.google")
+	if !errors.Is(err, io.EOF) {
+		t.Fatal("not the error we expected", err)
+	}
+	if out != nil {
+		t.Fatal("expected nil here")
+	}
+}