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
program code imports it. Every import and mention is updated to the new
name.

Model: opus-5-5

README.md

@@ -278,7 +278,7 @@ internal/
   tlscheck/tlscheck.go        TLS certificate inspector
   notify/notify.go             Notification service (Slack, Mattermost, ntfy)
   watcher/watcher.go           Main monitoring orchestrator and scheduler
-  livedns/livedns.go           Retry and concurrency limit for tests
+  livednstest/livednstest.go   Retry and concurrency limit for tests
                                against live DNS (imported only by tests)
 ```