tests: extend mount-ns test to cover Ubuntu 18.04 to 24.04

[zyga]

The test relies on a set of data files that contain expected contents of the mount table, transformed for determinism and observed from the point of view of a per snap and per snap-and-user mount namespace of a core16-based snap, core18-based snap and a classic snap.

[bboozzoo]

Can we get that landed after #14294 lands? Also, is there away to update/generate this data?

[zyga]

The test itself generates the data, all you have to do is copy it back.

[ernestl]

Based on feedback from @zyga, not important for milestone 2.67 - removing.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 79.03%. Comparing base (96ea7b0) to head (61a70aa). :warning: Report is 1285 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14407      +/-   ##
==========================================
+ Coverage   78.95%   79.03%   +0.08%     
==========================================
  Files        1084     1087       +3     
  Lines      146638   147700    +1062     
==========================================
+ Hits       115773   116735     +962     
- Misses      23667    23734      +67     
- Partials     7198     7231      +33     

Flag	Coverage Δ
unittests	79.03% <ø> (+0.08%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[zyga]

I'll close this as I don't see a lot of value in it. Perhaps after the new mount logic drops we can revisit.