ISO testing jobs seem to lack a timeout

https://jenkins.tails.boum.org/job/test_Tails_ISO_isotester4/27/ and https://jenkins.tails.boum.org/job/test_Tails_ISO_isotester3/23/ have been running for 4 days now. ISO testing jobs should not keep Jenkins slaves busy forever, and cleaning that up should not require manual intervention.

Parent Task: #5288 (closed)

Original created by @intrigeri on 10229 (Redmine)

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