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/geolocate/mmdblookup_test.go

@@ -0,0 +1,103 @@
+package geolocate
+
+import (
+	"context"
+	"net/http"
+	"testing"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/engine/resources"
+)
+
+const (
+	asnDBPath     = "../testdata/asn.mmdb"
+	countryDBPath = "../testdata/country.mmdb"
+	ipAddr        = "35.204.49.125"
+)
+
+func maybeFetchResources(t *testing.T) {
+	c := &resources.Client{
+		HTTPClient: http.DefaultClient,
+		Logger:     log.Log,
+		UserAgent:  "ooniprobe-engine/0.1.0",
+		WorkDir:    "../testdata/",
+	}
+	if err := c.Ensure(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestLookupASN(t *testing.T) {
+	maybeFetchResources(t)
+	asn, org, err := LookupASN(asnDBPath, ipAddr)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if asn <= 0 {
+		t.Fatal("unexpected ASN value")
+	}
+	if len(org) <= 0 {
+		t.Fatal("unexpected org value")
+	}
+}
+
+func TestLookupASNInvalidFile(t *testing.T) {
+	maybeFetchResources(t)
+	asn, org, err := LookupASN("/nonexistent", ipAddr)
+	if err == nil {
+		t.Fatal("expected an error here")
+	}
+	if asn != DefaultProbeASN {
+		t.Fatal("expected a zero ASN")
+	}
+	if org != DefaultProbeNetworkName {
+		t.Fatal("expected an empty org")
+	}
+}
+
+func TestLookupASNInvalidIP(t *testing.T) {
+	maybeFetchResources(t)
+	asn, org, err := LookupASN(asnDBPath, "xxx")
+	if err == nil {
+		t.Fatal("expected an error here")
+	}
+	if asn != DefaultProbeASN {
+		t.Fatal("expected a zero ASN")
+	}
+	if org != DefaultProbeNetworkName {
+		t.Fatal("expected an empty org")
+	}
+}
+
+func TestLookupCC(t *testing.T) {
+	maybeFetchResources(t)
+	cc, err := (mmdbLookupper{}).LookupCC(countryDBPath, ipAddr)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(cc) != 2 {
+		t.Fatal("does not seem a country code")
+	}
+}
+
+func TestLookupCCInvalidFile(t *testing.T) {
+	maybeFetchResources(t)
+	cc, err := (mmdbLookupper{}).LookupCC("/nonexistent", ipAddr)
+	if err == nil {
+		t.Fatal("expected an error here")
+	}
+	if cc != DefaultProbeCC {
+		t.Fatal("expected an empty cc")
+	}
+}
+
+func TestLookupCCInvalidIP(t *testing.T) {
+	maybeFetchResources(t)
+	cc, err := (mmdbLookupper{}).LookupCC(asnDBPath, "xxx")
+	if err == nil {
+		t.Fatal("expected an error here")
+	}
+	if cc != DefaultProbeCC {
+		t.Fatal("expected an empty cc")
+	}
+}