TestVerifySmimesignAttachedWithTimestamp 2028-01-18 time bomb

Hi. There is a time bomb in the self-checks that will trigger after 2028-01-18. We are going to disable this self-test in Debian to avoid it causing trouble. Could you consider doing something better here? Extending it 20 years more is one option, but maybe there are better solutions available.

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1091500

```
verify_test.go:159: x509: certificate has expired or is not yet valid: current time 2028-06-10T12:00:37Z is after 2028-01-18T00:00:00Z
--- FAIL: TestVerifySmimesignAttachedWithTimestamp (0.00s)
```

Thanks,
Simon

Inizia con verify_test.go e TestVerifySmimesignAttachedWithTimestamp, quindi esamina come il certificato o il timestamp del test sia fissato al 2028-01-18. Scegli un approccio duraturo che eviti la bomba a orologeria della scadenza segnalata e conferma che l'autoverifica abbia esito positivo oltre tale data.

Scritto dal modello di indicizzazione a partire dal testo della issue.