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
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