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
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