fix: enable gofumpt instead of gofmt linter in tests

[mmorel-35]

Description

enables and fixes gofumpt linter issues in tests directory

Related to #18955

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: mmorel-35
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[k8s-ci-robot]

Hi @mmorel-35. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.72%. Comparing base (2abb334) to head (85de27e).

Additional details and impacted files

see 20 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #18953      +/-   ##
==========================================
+ Coverage   68.66%   68.72%   +0.06%     
==========================================
  Files         420      420              
  Lines       35563    35563              
==========================================
+ Hits        24420    24442      +22     
+ Misses       9711     9694      -17     
+ Partials     1432     1427       -5     

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