Move s3/testing_utils.go

[lucasmrod]

Resolves #45220 (one of many PRs)

Testing

• QA'd all new/changed functionality manually.

Smoke tested upload/download of software installers, upload/download of bootstrap packages.

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[codecov]

Codecov Report

❌ Patch coverage is 77.41935% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.71%. Comparing base (cb32085) to head (404a302).

Files with missing lines	Patch %	Lines
server/datastore/s3/s3test/s3test.go	80.00%	5 Missing and 3 partials ⚠️
server/datastore/s3/s3.go	72.72%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@                                      Coverage Diff                                       @@
##           remove-more-testing-utils-from-production-binaries-round-2   #45362      +/-   ##
==============================================================================================
- Coverage                                                       66.72%   66.71%   -0.01%     
==============================================================================================
  Files                                                            2731     2731              
  Lines                                                          218495   218487       -8     
  Branches                                                        10696    10696              
==============================================================================================
- Hits                                                           145786   145766      -20     
- Misses                                                          59500    59506       +6     
- Partials                                                        13209    13215       +6     

Flag	Coverage Δ
backend	68.55% <77.41%> (-0.01%)	⬇️

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The base branch was changed.