test_Tails_ISO_testing job is not enabled during the freeze

Originally created by @intrigeri on #11499 (Redmine)

We’re in the freeze for 2.4, so our jenkins-jobs repo rightfully has build+test jobs for the testing branch. So far, so good. Except the test job doesn’t show up in Jenkins (while the build job does), which prevents our QA to be done precisely on the branch that would need it most currently. What’s wrong?

Setting >>normal prio + 2.4 target version, since the problem won’t be reproducible anymore as soon as 2.4 is released, so if we want to fix it for the next major release, we can only do it now.

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