refactor: use testify consistently

[kruskall]

Some old tests were using gotest.tools but most of them seem
to use testify.

[ghost]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-07-18T14:53:31.728+0000

• Duration: 20 min 10 sec

Test stats 🧪

Test	Results
Failed	0
Passed	224
Skipped	4
Total	228

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)