feat(netxlite): support extracting the CNAME (#875)

* feat(netxlite): support extracting the CNAME

Closes https://github.com/ooni/probe/issues/2225

* fix(netxlite): attempt to increase coverage and improve tests

1. dnsovergetaddrinfo: specify the behavior of a DNSResponse returned
by this file to make it line with normal responses and write unit tests
to make sure we adhere to expectations;

2. dnsoverudp: make sure we wait to deferred responses also w/o a
custom context and post on a private channel and test that;

3. utls: recognize that we can actually write a test for NetConn and
what needs to change when we'll use go1.19 by default will just be
a cast that at that point can be removed.

internal/model/mocks/dnsresponse.go

@@ -18,6 +18,7 @@ type DNSResponse struct {
 	MockDecodeHTTPS      func() (*model.HTTPSSvc, error)
 	MockDecodeLookupHost func() ([]string, error)
 	MockDecodeNS         func() ([]*net.NS, error)
+	MockDecodeCNAME      func() (string, error)
 }
 
 var _ model.DNSResponse = &DNSResponse{}
@@ -45,3 +46,7 @@ func (r *DNSResponse) DecodeLookupHost() ([]string, error) {
 func (r *DNSResponse) DecodeNS() ([]*net.NS, error) {
 	return r.MockDecodeNS()
 }
+
+func (r *DNSResponse) DecodeCNAME() (string, error) {
+	return r.MockDecodeCNAME()
+}

internal/model/mocks/dnsresponse_test.go

@@ -102,4 +102,20 @@ func TestDNSResponse(t *testing.T) {
 			t.Fatal("unexpected out")
 		}
 	})
+
+	t.Run("DecodeCNAME", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		r := &DNSResponse{
+			MockDecodeCNAME: func() (string, error) {
+				return "", expected
+			},
+		}
+		out, err := r.DecodeCNAME()
+		if !errors.Is(err, expected) {
+			t.Fatal("unexpected err", err)
+		}
+		if out != "" {
+			t.Fatal("unexpected out")
+		}
+	})
 }