Improve the automated tests about the included signing key

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

Original created by @tails on 5410 (Redmine)