tests: remove the DNS stand-ins from the watcher and resolver tests (closes #159)

The watcher tests used a stand-in resolver and the resolver timeout test
a stand-in DNS client, against the rule that DNS is never mocked.
Watcher tests that look something up in DNS now run the real resolver
against live servers, each attempt on a new watcher. A DNS change is
tested by saving values live DNS never returns (names under .invalid,
192.0.2.1) in the state a check starts from, or by marking a real
nameserver failed. The timeout test queries 192.0.2.1, where nothing
answers. The live-DNS retry and concurrency limit moved from the
resolver tests to internal/livedns, so both packages share them.
NewFromLoggerWithClient had no other use and is gone. TESTING.md now
states the README's rule.

Model: opus-5-5

internal/resolver/resolver_test.go

@@ -10,7 +10,6 @@ import (
 	"testing"
 	"time"
 
-	"github.com/miekg/dns"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
@@ -33,8 +32,8 @@ func newTestResolver(t *testing.T) *resolver.Resolver {
 }
 
 // findOneNSForDomain picks one authoritative nameserver to aim a
-// test at. Live-DNS retry, concurrency and quorum handling live in
-// livedns_test.go.
+// test at. Quorum handling lives in livedns_test.go, and the live-DNS
+// retry and concurrency limit in package livedns.
 func findOneNSForDomain(
 	t *testing.T,
 	r *resolver.Resolver,
@@ -526,21 +525,18 @@ func TestQueryAllNameservers_ContextCanceled(t *testing.T) {
 func TestQueryNameserverIP_Timeout(t *testing.T) {
 	t.Parallel()
 
-	log := slog.New(slog.NewTextHandler(
-		os.Stderr,
-		&slog.HandlerOptions{Level: slog.LevelDebug},
-	))
-
-	r := resolver.NewFromLoggerWithClient(
-		log, &timeoutClient{},
-	)
+	r := newTestResolver(t)
 
+	// Nothing answers at 192.0.2.1, a documentation address. The
+	// resolver tries each query twice, and the first try gives up
+	// after two seconds. A deadline that ends during the first try
+	// makes the status vary from run to run between nodata and
+	// timeout, so the deadline must outlast the first try.
 	ctx, cancel := context.WithTimeout(
-		context.Background(), 10*time.Second,
+		context.Background(), 3*time.Second,
 	)
 	t.Cleanup(cancel)
 
-	// Query any IP — the client always returns a timeout error.
 	resp, err := r.QueryNameserverIP(
 		ctx, "unreachable.test.", "192.0.2.1",
 		"example.com",
@@ -551,27 +547,6 @@ func TestQueryNameserverIP_Timeout(t *testing.T) {
 	assert.NotEmpty(t, resp.Error)
 }
 
-// timeoutClient simulates DNS timeout errors for testing.
-type timeoutClient struct{}
-
-func (c *timeoutClient) ExchangeContext(
-	_ context.Context,
-	_ *dns.Msg,
-	_ string,
-) (*dns.Msg, time.Duration, error) {
-	return nil, 0, &net.OpError{
-		Op:  "read",
-		Net: "udp",
-		Err: &timeoutError{},
-	}
-}
-
-type timeoutError struct{}
-
-func (e *timeoutError) Error() string   { return "i/o timeout" }
-func (e *timeoutError) Timeout() bool   { return true }
-func (e *timeoutError) Temporary() bool { return true }
-
 func TestResolveIPAddresses_ContextCanceled(t *testing.T) {
 	t.Parallel()