[join] renew testbed certificate

[yoursunny]

If the user's testbed certificate has expired, the error says "failed to register prefixes" but does not point to the real reason.

The application does not display the expiration date of the testbed certificate.
How is the user supposed to know the failure is caused by certificate expiration?

The application should suggest renewing / re-obtaining the testbed certificate, well before it's expired, not after it's expired.
Reference: acme.sh renews 30 days prior. Since testbed certificate is only valid for 14 days, maybe renew 2 days prior?