tests: detect loop devices with deleted files as an invariant

[zyga]

When a loop device is created with a given backing file, the file itself may be deleted. The kernel keeps the reference to the file alive but it also adjusts the name of the file to have the suffix " (deleted)".

We should not have loop devices with this property across tests. They might indicate that a mount sticks around and leaks through a reference somehow, or that the low-level loop-back utilities are buggy.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :warning: Please upload report for BASE (master@f1825ad). Learn more about missing BASE report.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #14568   +/-   ##
=========================================
  Coverage          ?   77.44%           
=========================================
  Files             ?     1318           
  Lines             ?   178396           
  Branches          ?     2332           
=========================================
  Hits              ?   138162           
  Misses            ?    31808           
  Partials          ?     8426           

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unittests	77.44% <ø> (?)

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[github-actions[bot]]

Fri Sep 19 11:21:21 UTC 2025 The following results are from: https://github.com/canonical/snapd/actions/runs/17854769010

Failures:

Preparing:

• openstack-ps7:ubuntu-core-24-64:tests/main/apparmor-prompting-support
• openstack-ps7:ubuntu-24.04-64:tests/main/nvidia-userspace-libs

Executing:

• google-core:ubuntu-core-18-64:tests/core/kernel-base-gadget-pair-single-reboot:kernel_base
• openstack-ps7:ubuntu-core-24-64:tests/main/dbus-activation-session-legacy
• openstack-ps7:ubuntu-core-24-64:tests/main/dbus-activation-name-conflict
• openstack-ps7:ubuntu-core-24-64:tests/main/dbus-activation-session
• openstack-ps7:ubuntu-25.04-64:tests/main/try

Restoring:

• google-core:ubuntu-core-18-64:tests/core/kernel-base-gadget-pair-single-reboot:kernel_base
• google-core:ubuntu-core-18-64:tests/core/
• google-core:ubuntu-core-18-64
• google-core:ubuntu-core-18-64:tests/core/remodel-gadget
• google-core:ubuntu-core-18-64:tests/core/auto-refresh-backoff-after-reboot:kernel
• openstack-ps7:ubuntu-core-20-64:tests/core/snapd-refresh-vs-services:start_w_2_49_2
• openstack-ps7:ubuntu-core-20-64:tests/core/gadget-update-pc
• openstack-ps7:ubuntu-core-24-64:tests/core/interfaces-gpio-chardev
• openstack-ps7:ubuntu-core-24-64:tests/core/gadget-update-pc

[zyga]

There is something wrong with that core24 gadget snap test. I will investigate locally.