tests: small improvements to mounts persist refresh content

[zyga]

A few small improvements to the test I've been looking at for the past few days.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 77.31%. Comparing base (06f7fe7) to head (c2f6ee0). :warning: Report is 47 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14722      +/-   ##
==========================================
- Coverage   77.37%   77.31%   -0.07%     
==========================================
  Files        1301     1312      +11     
  Lines      175641   175866     +225     
  Branches     2332     2332              
==========================================
+ Hits       135900   135967      +67     
- Misses      31399    31546     +147     
- Partials     8342     8353      +11     

Flag	Coverage Δ
unittests	77.31% <ø> (-0.07%)	:arrow_down:

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[github-actions[bot]]

Thu Aug 14 08:06:41 UTC 2025 The following results are from: https://github.com/canonical/snapd/actions/runs/16957432166

Failures:

Preparing:

• openstack:opensuse-tumbleweed-64
• openstack:opensuse-tumbleweed-selinux-64

Restoring:

• openstack:opensuse-tumbleweed-64
• openstack:opensuse-tumbleweed-selinux-64