refactor: replace gotest.tools assertions with testify

[kruskall]

Proposed commit message

gotest.tools assertions are only used in a few files. Switch to testify to drop gotest.tools dependency

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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[mergify[bot]]

This pull request does not have a backport label. If this is a bug or security fix, could you label this PR @kruskall? 🙏. 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

[kruskall]

That's a lot of reviewers, sorry for the noise! :sweat:

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