test: unified testimages

[phbelitz]

Unified the building process of all testimages to avoid unnecessary code duplication.

Checklist

• PR is rebased to/aimed at branch develop
• PR follows Contributing Guide
• Added tests (if necessary)
• Extended README/Documentation (if necessary)
• Adjusted versions of image and Helm chart in Chart.yaml (if necessary)

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.00%. Comparing base (1562cd9) to head (180d77e).
Report is 27 commits behind head on develop.

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1697      +/-   ##
===========================================
- Coverage    88.08%   84.00%   -4.08%     
===========================================
  Files           36       42       +6     
  Lines         2509     2939     +430     
===========================================
+ Hits          2210     2469     +259     
- Misses         255      404     +149     
- Partials        44       66      +22     

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