Test suite: relax delay for OpenPGP future expiration date

Sysadmins will implement a monitoring check (sysadmin#17816 (closed)) that will give them an advance warning to update these keys.

And then, this very test scenario will be about whether we did import the key that was already updated by sysadmins.

This change avoids this scenario failing, potentially for a long time, until sysadmins update the key, which is currently the case since 3 weeks.

I think we should do the same for Tails' APT repository keys, but let's first ensure sysadmins will monitor this key as well.

refs #18170 (closed)