USB image test sometimes fails on Jenkins with "No space left"
Originally created by @segfault on #17626 (Redmine)
See for example https://jenkins.tails.boum.org/job/test_Tails_ISO_devel/2039/
Error creating disk "usbimage" in "/tmp/TailsToaster/TailsToasterStorage". Need 7200 MiB but only 7695 MiB is available of which 500 MiB is reserved for other temporary files.
<false> is not true. (Test::Unit::AssertionFailedError)
./features/support/helpers/storage_helper.rb:99:in `create_new_disk'
./features/step_definitions/common_steps.rb:65:in `/^I (temporarily )?create an? (\d+) ([[:alpha:]]+) (?:([[:alpha:]]+) )?disk named "([^"]+)"$/'
features/usb_install.feature:128:in `And I create a 7200 MiB disk named "usbimage"'