test: add unit tests for argocd admin export command to validate behaviour

[nitishfy]

Completes 1st part of #21895
(The import command code needs a lot of refactoring so I thought it's best to raise those tests in another PR to avoid difficulty in the review process)

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• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 167 lines in your changes missing coverage. Please review.

Project coverage is 55.87%. Comparing base (fe347e3) to head (3267631).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/argocd/commands/admin/backup.go	0.00%	167 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #22391      +/-   ##
==========================================
- Coverage   55.88%   55.87%   -0.01%     
==========================================
  Files         343      343              
  Lines       57333    57382      +49     
==========================================
+ Hits        32042    32065      +23     
- Misses      22644    22670      +26     
  Partials     2647     2647              

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[rumstead]

Instead of passing in a string "out", should the user of these functions pass in an io.Writer? This function also wouldn't need to have any cleanup logic then.