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/netxlite/dnsdecoder.go

@@ -166,5 +166,19 @@ func (r *dnsResponse) DecodeNS() ([]*net.NS, error) {
 	return out, nil
 }
 
+// DecodeCNAME implements model.DNSResponse.DecodeCNAME.
+func (r *dnsResponse) DecodeCNAME() (string, error) {
+	if err := r.rcodeToError(); err != nil {
+		return "", err
+	}
+	for _, answer := range r.msg.Answer {
+		switch avalue := answer.(type) {
+		case *dns.CNAME:
+			return avalue.Target, nil
+		}
+	}
+	return "", ErrOODNSNoAnswer
+}
+
 var _ model.DNSDecoder = &DNSDecoderMiekg{}
 var _ model.DNSResponse = &dnsResponse{}