Test suite fails for feature/stretch on Jenkins due to lack of space in /tmp/TailsToaster

https://jenkins.tails.boum.org/view/RM/job/test_Tails_ISO_feature-stretch/74/consoleFull says:

18:29:48 Feature: Icedove email client
18:29:48   As a Tails user
18:29:48   I may want to use an email client
18:29:48
18:29:48   Background:                                                                      # features/icedove.feature:7
18:30:37     Given I have started Tails from DVD and logged in and the network is connected # features/step_definitions/snapshots.rb:197
18:30:37       Call to virDomainSaveFlags failed: operation failed: domain save job: unexpectedly failed (Libvirt::Error)
18:30:37       ./features/support/helpers/vm_helper.rb:589:in `save'
18:30:37       ./features/support/helpers/vm_helper.rb:589:in `save_snapshot'
18:30:37       ./features/step_definitions/snapshots.rb:190:in `reach_checkpoint'
18:30:37       ./features/step_definitions/snapshots.rb:198:in `/^I\ have\ started\ Tails\ from\ DVD\ and\ logged\ in\ and\ the\ network\ is\ connected$/'
18:30:37       features/icedove.feature:8:in `Given I have started Tails from DVD and logged in and the network is connected'

Around that time, thanks to home-made monitoring:

$ df $df_options
Filesystem     Type   Size  Used Avail Use% Mounted on
tmpfs          tmpfs   13G   13G 1017M  93% /tmp/TailsToaster

$ free $free_options
             total       used       free     shared    buffers     cached
Mem:         23107      20664       2443      12310         61      14093
-/+ buffers/cache:       6509      16597
Swap:            0          0          0

So, for some reason, running the test suite on current feature/stretch requires more disk space (and probably more RAM).
I’ll investigate why (it might be caused by the changes I did for #12055 (closed)).

Related issues

• Related to #12092 (closed)

Original created by @intrigeri on 12088 (Redmine)