watcher tests: far fewer live queries, longer live attempts (closes #214)

A domain check looked up each nameserver's addresses by asking every
nameserver of that name's zone for all eight record types; it now asks
only for A, AAAA and CNAME, the ones it reads.

The watcher tests now check example.org instead of cloudflare.com (two
nameservers instead of five) and desec.io instead of example.com (its
nameservers are in zones with two, not cloudflare.com's five). The
record change and NS failure tests start from saved state built on one
NS lookup instead of a first full check, and the port change test runs
only the port checks again. A live test attempt may take 18 seconds,
not 8. A new live test checks that a nameserver's addresses include
IPv4 and IPv6.

Model: opus-5-5

internal/livednstest/livednstest.go

@@ -36,11 +36,18 @@ const (
 	// before the test fails.
 	attempts = 3
 
-	// AttemptTimeout bounds one attempt. Worst case for an operation
-	// is attempts * AttemptTimeout plus the backoff — about 26
-	// seconds, well inside the 90-second `go test -timeout` backstop
-	// even when several operations exhaust their attempts.
-	AttemptTimeout = 8 * time.Second
+	// AttemptTimeout bounds one attempt. It must fit the longest
+	// operation, a watcher check, which sends over a hundred queries one
+	// after another and on a slow build host takes several times as long
+	// as the few seconds it takes on a fast one. An operation whose
+	// every attempt fails takes attempts * AttemptTimeout plus the
+	// backoff, about 56 seconds, after it waits for one of the
+	// Concurrency slots that every live operation in the test binary
+	// shares. So when live DNS does not answer at all, a test binary
+	// with more live operations than slots runs into the 90-second
+	// `go test -timeout` backstop instead of each test failing on its
+	// own.
+	AttemptTimeout = 18 * time.Second
 
 	// backoffBase is the delay after the first failed attempt; it is
 	// multiplied by backoffFactor each time.