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

.golangci.yml

@@ -60,6 +60,8 @@ linters:
               desc: >-
                 Test-support code belongs in test files and in packages whose
                 directory name ends in test, not in the shipped binary.
+            - pkg: sneak.berlin/go/dnswatcher/internal/livednstest
+              desc: Live-DNS test support belongs in test files only.
     # Only decisions already recorded in the Go package defaults are
     # listed here. Every entry matches the module path exactly.
     gomodguard_v2: