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/netx.go

@@ -36,6 +36,9 @@ type DNSResponse interface {
 
 	// DecodeNS returns all the NS entries in this response.
 	DecodeNS() ([]*net.NS, error)
+
+	// DecodeCNAME returns the first CNAME entry in this response.
+	DecodeCNAME() (string, error)
 }
 
 // The DNSDecoder decodes DNS responses.