test suite: fix persistence is not enabled check
_Originally created by Tails on [#5465 (Redmine)](https://public-redmine-archive.tails.boum.org/code/issues/5465)_
This check is incorrect:
data_part_dev = boot_device + "2"
assert(!@vm.execute("grep -q '^#{data_part_dev} ' /proc/mounts").success?,
"Partition '#{data_part_dev}' from the boot device is mounted")
I believe it will success even if persistence is enabled.
Feature Branch: test/fix-persistence-checks
Parent Task: tails/tails#5978
### Related issues
- [x] **Blocked by** tails/tails#6399
issue