refactor(oohelperd): flatten package hierarchy (#834)

In https://github.com/ooni/probe-cli/pull/832's initial diff, I
mentioned it would be cool to flatten oohelperd's hier.

I'm doing this now, and just for the master branch.

This diff is mostly a mechanical refactoring with very light
and apparently rather safe manual changes.

internal/cmd/oohelperd/dns_test.go

@@ -0,0 +1,93 @@
+package main
+
+import (
+	"context"
+	"sync"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity"
+	"github.com/ooni/probe-cli/v3/internal/model"
+	"github.com/ooni/probe-cli/v3/internal/model/mocks"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+)
+
+func stringPointerForString(s string) *string {
+	return &s
+}
+
+func Test_dnsMapFailure(t *testing.T) {
+	tests := []struct {
+		name    string
+		failure *string
+		want    *string
+	}{{
+		name:    "nil",
+		failure: nil,
+		want:    nil,
+	}, {
+		name:    "nxdomain",
+		failure: stringPointerForString(netxlite.FailureDNSNXDOMAINError),
+		want:    stringPointerForString(webconnectivity.DNSNameError),
+	}, {
+		name:    "no answer",
+		failure: stringPointerForString(netxlite.FailureDNSNoAnswer),
+		want:    nil,
+	}, {
+		name:    "non recoverable failure",
+		failure: stringPointerForString(netxlite.FailureDNSNonRecoverableFailure),
+		want:    stringPointerForString("dns_server_failure"),
+	}, {
+		name:    "refused",
+		failure: stringPointerForString(netxlite.FailureDNSRefusedError),
+		want:    stringPointerForString("dns_server_failure"),
+	}, {
+		name:    "server misbehaving",
+		failure: stringPointerForString(netxlite.FailureDNSServerMisbehaving),
+		want:    stringPointerForString("dns_server_failure"),
+	}, {
+		name:    "temporary failure",
+		failure: stringPointerForString(netxlite.FailureDNSTemporaryFailure),
+		want:    stringPointerForString("dns_server_failure"),
+	}, {
+		name:    "anything else",
+		failure: stringPointerForString(netxlite.FailureEOFError),
+		want:    stringPointerForString("unknown_error"),
+	}}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := dnsMapFailure(tt.failure)
+			if diff := cmp.Diff(tt.want, got); diff != "" {
+				t.Fatal(diff)
+			}
+		})
+	}
+}
+
+func TestDNSDo(t *testing.T) {
+	t.Run("returns non-nil addresses list on nxdomin", func(t *testing.T) {
+		ctx := context.Background()
+		config := &dnsConfig{
+			Domain: "antani.ooni.org",
+			NewResolver: func() model.Resolver {
+				return &mocks.Resolver{
+					MockLookupHost: func(ctx context.Context, domain string) ([]string, error) {
+						return nil, netxlite.ErrOODNSNoSuchHost
+					},
+					MockCloseIdleConnections: func() {
+						// nothing
+					},
+				}
+			},
+			Out: make(chan webconnectivity.ControlDNSResult, 1),
+			Wg:  &sync.WaitGroup{},
+		}
+		config.Wg.Add(1)
+		dnsDo(ctx, config)
+		config.Wg.Wait()
+		resp := <-config.Out
+		if resp.Addrs == nil || len(resp.Addrs) != 0 {
+			t.Fatal("returned nil Addrs or Addrs containing replies")
+		}
+	})
+}