Update the memory erasure automated tests for Jessie

Originally created by @intrigeri on #9705 (Redmine)

I see two of its scenarios fail due to insufficient patterns coverage after filling RAM with a known pattern. We should first check that we correctly measure this (e.g. I’ve fixed some of it with 505297c8 already, and there may be more to update). If we don’t, then fix it. If we do, then assess whether it’s expected that on Jessie, less patterns are found.

Feature Branch: feature/jessie

Parent Task: #7563 (closed)

Related issues

• Related to #10487 (closed)
• Blocked by #8317 (closed)