Improve the automated tests about the included signing key
Originally created by Tails on #5410 (Redmine)
Why test "old w.r.t. the one we fetch from Tails’ website"? What we mainly need to test here (and the reason why this test was added to begin with) is that the key and its subkeys haven’t expired, and won’t expire any time soon.
Feature Branch: kytv:test/5410-improve-signing-key-test