Jessie: System sometimes does not poweroff after memory erasure

Reproduced in VMs and via the I shutdown Tails and wait for the computer to power off test suite step: I see a bunch of stars (sdmem progress) then it stops and nothing happens anymore.

Feature Branch: test/9707-power-off-after-memory-erasure

Related issues

Related to #10487 (closed)
Related to #11786 (closed)
Blocked by #10733 (closed)

Original created by @intrigeri on 9707 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information