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. The
watcher tests now run the real resolver against live DNS. A 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 and
the DNSClient comment now state the README's rule.

Model: opus-5-5

internal/livedns/livedns.go

@@ -16,10 +16,12 @@
 //     operations are in flight at once in one test binary.
 //
 //  2. Retry with exponential backoff. Each live operation gets several
-//     attempts with its own timeout. The retry condition is strictly
+//     attempts with its own timeout. An attempt is retried when it
-//     transport-level — "did a nameserver answer at all" — never the
+//     obtained nothing to check, never because of what the test
-//     assertion the test is making. Code that answers incorrectly
+//     asserts about the result, so a wrong result still fails on the
-//     still fails on the first attempt.
+//     first attempt. A fault in the code under test that leaves
+//     nothing to check looks the same as live DNS not answering, and
+//     fails only after the last attempt.
 package livedns
 
 import (
@@ -113,7 +115,7 @@ func Retry(
 	}
 
 	t.Fatalf(
-		"%s: no answer after %d live attempts: %v",
+		"%s: all %d live attempts failed: %v",
 		what, attempts, last,
 	)
 }

internal/watcher/watcher_test.go

@@ -171,11 +171,11 @@ func defaultTestConfig(t *testing.T) *config.Config {
 	}
 }
 
-// checkOnce runs the watcher's checks once and returns
+// checkOnce runs the watcher's checks once and returns an error when a
-// livedns.ErrNoAnswer when live DNS did not answer for a configured
+// configured name has no hostname state saved by this check, or that
-// name. The watcher saves a name's hostname state only when all of the
+// state holds no address. The watcher saves a name's hostname state
-// name's lookups succeed, so live DNS answered for a name when this
+// only when all of the name's lookups succeed, so this means either
-// check saved its hostname state and that state holds an address.
+// live DNS did not answer or the watcher did not save what it got.
 func checkOnce(
 	ctx context.Context,
 	w *watcher.Watcher,
@@ -191,7 +191,11 @@ func checkOnce(
 		hs, ok := deps.state.GetHostnameState(name)
 		if !ok || hs.LastChecked.Before(started) ||
 			len(addresses(hs)) == 0 {
-			return fmt.Errorf("%w: %s", livedns.ErrNoAnswer, name)
+			return fmt.Errorf(
+				"%s: %w, or the watcher saved no fresh "+
+					"result for it",
+				name, livedns.ErrNoAnswer,
+			)
 		}
 	}