watcher: save and notify nothing for a cut-short port or TLS check (closes #185)

When shutdown cancels a check that is under way, the rest of the check
still runs with the cancelled context. The resolver already drops a
lookup the context cut short, but a cancelled connection attempt was
saved as a closed port or a failed certificate check and notified as
Port Change or TLS Failure. The watcher now drops a port or TLS check
result when its context was cancelled, the same way. The test runs a
check with the context already cancelled, using the real resolver and
the real port and TLS checkers; no query is sent and no connection is
made.

Model: opus-5-5

internal/watcher/watcher.go

@@ -658,6 +658,13 @@ func (w *Watcher) checkSinglePort(
 	hostnames []string,
 ) {
 	result, err := w.portCheck.CheckPort(ctx, ip, port)
+
+	// A check the context cut short says nothing about the port, so it
+	// is neither saved nor notified.
+	if ctx.Err() != nil {
+		return
+	}
+
 	if err != nil {
 		w.log.Error(
 			"port check failed",
@@ -733,6 +740,13 @@ func (w *Watcher) checkTLSCert(
 	hostname string,
 ) {
 	cert, err := w.tlsCheck.CheckCertificate(ctx, ip, hostname)
+
+	// A check the context cut short says nothing about the certificate,
+	// so it is neither saved nor notified.
+	if ctx.Err() != nil {
+		return
+	}
+
 	certKey := fmt.Sprintf("%s:%d:%s", ip, tlsPort, hostname)
 	now := time.Now().UTC()
 	prev, hasPrev := w.state.GetCertificateState(certKey)