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 four places in the watcher
that log a failed lookup, checkDomain, checkHostname,
resolveNameserverAddresses and resolveCNAMEAddresses, now do it through
logFailedLookup, which logs nothing when the watcher's context was
cancelled. It asks the context, not the lookup's error: 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. The tests run each of these
lookups 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,