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/netx/resolver/dnsoverhttps_test.go

@@ -0,0 +1,165 @@
+package resolver_test
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"io/ioutil"
+	"net/http"
+	"strings"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/httpheader"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/resolver"
+)
+
+func TestDNSOverHTTPSNewRequestFailure(t *testing.T) {
+	const invalidURL = "\t"
+	txp := resolver.NewDNSOverHTTPS(http.DefaultClient, invalidURL)
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if err == nil || !strings.HasSuffix(err.Error(), "invalid control character in URL") {
+		t.Fatal("expected an error here")
+	}
+	if data != nil {
+		t.Fatal("expected no response here")
+	}
+}
+
+func TestDNSOverHTTPSClientDoFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	txp := resolver.DNSOverHTTPS{
+		Do: func(*http.Request) (*http.Response, error) {
+			return nil, expected
+		},
+		URL: "https://cloudflare-dns.com/dns-query",
+	}
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if !errors.Is(err, expected) {
+		t.Fatal("expected an error here")
+	}
+	if data != nil {
+		t.Fatal("expected no response here")
+	}
+}
+
+func TestDNSOverHTTPSHTTPFailure(t *testing.T) {
+	txp := resolver.DNSOverHTTPS{
+		Do: func(*http.Request) (*http.Response, error) {
+			return &http.Response{
+				StatusCode: 500,
+				Body:       ioutil.NopCloser(strings.NewReader("")),
+			}, nil
+		},
+		URL: "https://cloudflare-dns.com/dns-query",
+	}
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if err == nil || err.Error() != "doh: server returned error" {
+		t.Fatal("expected an error here")
+	}
+	if data != nil {
+		t.Fatal("expected no response here")
+	}
+}
+
+func TestDNSOverHTTPSMissingContentType(t *testing.T) {
+	txp := resolver.DNSOverHTTPS{
+		Do: func(*http.Request) (*http.Response, error) {
+			return &http.Response{
+				StatusCode: 200,
+				Body:       ioutil.NopCloser(strings.NewReader("")),
+			}, nil
+		},
+		URL: "https://cloudflare-dns.com/dns-query",
+	}
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if err == nil || err.Error() != "doh: invalid content-type" {
+		t.Fatal("expected an error here")
+	}
+	if data != nil {
+		t.Fatal("expected no response here")
+	}
+}
+
+func TestDNSOverHTTPSSuccess(t *testing.T) {
+	body := []byte("AAA")
+	txp := resolver.DNSOverHTTPS{
+		Do: func(*http.Request) (*http.Response, error) {
+			return &http.Response{
+				StatusCode: 200,
+				Body:       ioutil.NopCloser(bytes.NewReader(body)),
+				Header: http.Header{
+					"Content-Type": []string{"application/dns-message"},
+				},
+			}, nil
+		},
+		URL: "https://cloudflare-dns.com/dns-query",
+	}
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(data, body) {
+		t.Fatal("not the response we expected")
+	}
+}
+
+func TestDNSOverHTTPTransportOK(t *testing.T) {
+	const queryURL = "https://cloudflare-dns.com/dns-query"
+	txp := resolver.NewDNSOverHTTPS(http.DefaultClient, queryURL)
+	if txp.Network() != "doh" {
+		t.Fatal("invalid network")
+	}
+	if txp.RequiresPadding() != true {
+		t.Fatal("should require padding")
+	}
+	if txp.Address() != queryURL {
+		t.Fatal("invalid address")
+	}
+}
+
+func TestDNSOverHTTPSClientSetsUserAgent(t *testing.T) {
+	expected := errors.New("mocked error")
+	var correct bool
+	txp := resolver.DNSOverHTTPS{
+		Do: func(req *http.Request) (*http.Response, error) {
+			correct = req.Header.Get("User-Agent") == httpheader.UserAgent()
+			return nil, expected
+		},
+		URL: "https://cloudflare-dns.com/dns-query",
+	}
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if !errors.Is(err, expected) {
+		t.Fatal("expected an error here")
+	}
+	if data != nil {
+		t.Fatal("expected no response here")
+	}
+	if !correct {
+		t.Fatal("did not see correct user agent")
+	}
+}
+
+func TestDNSOverHTTPSHostOverride(t *testing.T) {
+	var correct bool
+	expected := errors.New("mocked error")
+
+	hostOverride := "test.com"
+	txp := resolver.DNSOverHTTPS{
+		Do: func(req *http.Request) (*http.Response, error) {
+			correct = req.Host == hostOverride
+			return nil, expected
+		},
+		URL:          "https://cloudflare-dns.com/dns-query",
+		HostOverride: hostOverride,
+	}
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if !errors.Is(err, expected) {
+		t.Fatal("expected an error here")
+	}
+	if data != nil {
+		t.Fatal("expected no response here")
+	}
+	if !correct {
+		t.Fatal("did not see correct host override")
+	}
+}