watcher: a lookup cut short by shutdown is not logged as an error (closes #229)

Stopping dnswatcher during a DNS check logged every lookup the stop cut
short as an error, "context canceled". The watcher now logs a failed
lookup only when its context was not cancelled; the port and TLS checks
already return quietly. It asks the context, not the lookup's error,
because the resolver reports a cancelled lookup with its own error,
which does not wrap context.Canceled. A context whose deadline passed is
not cancelled, so a lookup that ran out of time is still logged as an
error. The tests run a check on a cancelled context and on one whose
deadline passed; neither sends a query.

Model: opus-5-5

internal/watcher/export_test.go

@@ -31,6 +31,12 @@ func NewForTest(
 	}
 }
 
+// SetLogger replaces the watcher's logger, so a test can read what it
+// logs.
+func (w *Watcher) SetLogger(log *slog.Logger) {
+	w.log = log
+}
+
 // NewlyDisagreeingPairs exports newlyDisagreeingPairs for testing.
 func NewlyDisagreeingPairs(
 	prev, current *state.HostnameState,