tests: rename internal/livedns to livednstest and deny it outside tests (closes #164)

The live-DNS retry and concurrency limit is only for tests, but nothing
stopped program code from importing it and compiling it into the
binary. Its directory name now ends in test, and its import path is on
the test-support deny list in .golangci.yml, so make lint fails when a
file that is not a test imports it. Every import and mention is updated
to the new name.

Model: opus-5-5

internal/watcher/watcher_test.go

@@ -10,7 +10,7 @@ import (
 	"time"
 
 	"sneak.berlin/go/dnswatcher/internal/config"
-	"sneak.berlin/go/dnswatcher/internal/livedns"
+	"sneak.berlin/go/dnswatcher/internal/livednstest"
 	"sneak.berlin/go/dnswatcher/internal/portcheck"
 	"sneak.berlin/go/dnswatcher/internal/resolver"
 	"sneak.berlin/go/dnswatcher/internal/state"
@@ -195,7 +195,7 @@ func checkOnce(
 			return fmt.Errorf(
 				"%s: %w, or the watcher saved no fresh "+
 					"result for it",
-				name, livedns.ErrNoAnswer,
+				name, livednstest.ErrNoAnswer,
 			)
 		}
 	}
@@ -219,7 +219,7 @@ func runChecks(
 
 	var deps *testDeps
 
-	livedns.Retry(t, "watcher checks", func(ctx context.Context) error {
+	livednstest.Retry(t, "watcher checks", func(ctx context.Context) error {
 		var w *watcher.Watcher
 
 		w, deps = newTestWatcher(t, cfg)