test suite: fix persistence is not enabled check

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: #5978 (closed)

Related issues

• Blocked by #6399 (closed)

Original created by @tails on 5465 (Redmine)