Test suite: does not wait long enough for PersistentStorageNotUnlocked.png

The following discussion from !1644 (merged) should be addressed:

@role-jenkins-isotester started a discussion: (+6 comments)
Test suite run #1 of adfc5ef5 FAILURE: 243/244 scenarios passed

Failing scenarios
- features/erase_memory.feature:13 # Erasure of memory freed by killed userspace processes
debug.log

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