test_Tails_ISO_* fail on Jenkins due to lack of disk space

Originally created by @intrigeri on #16160 (Redmine)

I guess that’s because we now retrieve the .img too. I think the lack of disk space is in the /tmp/TailsToaster tmpfs (btw I was wrong yesterday, we do tmpfs instead of disk cache + ext4), to be confirmed though. So I guess we need to bump the size of this tmpfs and the isotester’s RAM accordingly.

Parent Task: #15993 (closed)