Dogtail optimizations breaks many tests on Stretch

Originally created by @intrigeri on #12305 (Redmine)

https://jenkins.tails.boum.org/view/RM/job/test_Tails_ISO_feature-stretch/188/ (run on an ISO built from 7837bcbf) was OK. https://jenkins.tails.boum.org/view/RM/job/test_Tails_ISO_feature-stretch/189/ (run on an ISO built from 73dc75ac) started to be much more broken. Meanwhile, test/12059-dogtail-optimizations was merged.

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