Make up our mind wrt. what old ISO our test suite should use on Jenkins

https://git-tails.immerda.ch/puppet-tails/commit/files/jenkins/slaves/isotesters/wrap_test_suite?id=6f18e697ad102b18de44f391ee38f78e205222eb was meant to be a temporary measure, to be reverted after the 2.7 release, but I could not find where the follow-ups are tracked, and I wonder if there was a good reason not to revert it, hence this ticket.

So, do we want to pass the last released ISO to --old-iso when testing release branches, or do we want to use the default (i.e. use the to-be-tested ISO)?

Related issues

Blocks #13239 (closed)

Original created by @intrigeri on 12299 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information