better antitesting of memory erasure
_Originally created by Tails on [#5959 (Redmine)](https://public-redmine-archive.tails.boum.org/code/issues/5959)_
code the anti-test of checking that, without erasing memory, we can
retrieve the known pattern. Just as we [do
manually](https://tails.boum.org/contribute/release_process/test/erase_memory_on_shutdown/).
Feature Branch: test/5959-antitest-memory-erasure
Parent Task: tails/tails#5690
### Related issues
- **Blocks** tails/tails#6106
- [x] **Blocked by** tails/tails#6399
issue