Don't leak testing packages into main

[axw]

stretchr/testify is unintentionally and unnecessarily being imported by a package that's part of the libbeat public API. Fix that by making the code only part of the test package.

Fixes elastic/apm-server#12580

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @axw? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

💚 Build Succeeded

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• Commit: e562f92

cc @axw

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[elasticmachine]

💚 Build Succeeded

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• Start Time: 2024-02-09T07:21:06.943+0000

• Duration: 142 min 31 sec

Test stats 🧪

Test	Results
Failed	0
Passed	28853
Skipped	2018
Total	30871

💚 Flaky test report

Tests succeeded.

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[ycombinator]

Thanks for finding and fixing!