snap-update-ns: add missing unit test for desired/current profile handling

[mvo5]

The cmd/snap-update-ns/main.go is a bit under-tested. This PR adds a direct unit test for the mount profile handling. It is a single test for now that tests the fix for #4139, however it should be easy to expand to test more conditions.

[zyga]

Looks nice, thank you mvo

[zyga]

I merged master, this should be mergable now

[codecov-io]

Codecov Report

Merging #4141 into master will increase coverage by 0.11%.
The diff coverage is 40%.

@@            Coverage Diff            @@
##           master   #4141      +/-   ##
=========================================
+ Coverage   75.38%   75.5%   +0.11%     
=========================================
  Files         435     435              
  Lines       37724   37731       +7     
=========================================
+ Hits        28439   28489      +50     
+ Misses       7296    7248      -48     
- Partials     1989    1994       +5

Impacted Files	Coverage Δ
cmd/snap-update-ns/main.go	38.46% <40%> (+38.46%)	⬆️

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