Automatically test that any administration password is not leaked in plaintext into the system

Originally created by @anonym on #10275 (Redmine)

We manually test that $TAILS_USER_PASSWORD is not set in some (shell) environments, but we could perhaps take it one step further by dumping the testing VMs memory and grep the dump for the password? I’m not sure how valid that is vs encodings, etc. so I guess we’d need an anti-test verifying that the approach works for other environment variables.

Parent Task: #10250 (closed)