chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/experiment/webconnectivity/dnslookup_test.go

@@ -0,0 +1,79 @@
+package webconnectivity_test
+
+import (
+	"context"
+	"net"
+	"net/url"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/urlgetter"
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity"
+)
+
+func TestDNSLookup(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skip test in short mode")
+	}
+	config := webconnectivity.DNSLookupConfig{
+		Session: newsession(t, true),
+		URL:     &url.URL{Host: "dns.google"},
+	}
+	out := webconnectivity.DNSLookup(context.Background(), config)
+	if out.Failure != nil {
+		t.Fatal(*out.Failure)
+	}
+	if len(out.Addrs) < 1 {
+		t.Fatal("no addresses?!")
+	}
+	for addr, asn := range out.Addrs {
+		if net.ParseIP(addr) == nil {
+			t.Fatal("invalid addr")
+		}
+		if asn != 15169 {
+			t.Fatal("invalid asn")
+		}
+	}
+	if len(out.TestKeys.NetworkEvents) < 1 {
+		t.Fatal("no network events?!")
+	}
+	if len(out.TestKeys.Queries) < 1 {
+		t.Fatal("no queries?!")
+	}
+}
+
+func TestDNSLookupResult_Addresses(t *testing.T) {
+	type fields struct {
+		Addrs    map[string]int64
+		Failure  *string
+		TestKeys urlgetter.TestKeys
+	}
+	tests := []struct {
+		name    string
+		fields  fields
+		wantOut []string
+	}{{
+		name:    "with no entries",
+		fields:  fields{},
+		wantOut: []string{},
+	}, {
+		name: "with some entries",
+		fields: fields{
+			Addrs: map[string]int64{"1.1.1.1": 1, "2001:4860:4860::8844": 2},
+		},
+		wantOut: []string{"1.1.1.1", "2001:4860:4860::8844"},
+	}}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			r := webconnectivity.DNSLookupResult{
+				Addrs:    tt.fields.Addrs,
+				Failure:  tt.fields.Failure,
+				TestKeys: tt.fields.TestKeys,
+			}
+			gotOut := r.Addresses()
+			if diff := cmp.Diff(tt.wantOut, gotOut); diff != "" {
+				t.Fatal(diff)
+			}
+		})
+	}
+}