Move testhelpers' code to _test.go file

[dmlambea]

Fixes #1048

[cla-assistant]

All committers have signed the CLA.

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[milindl]

Thanks for this change @dmlambea
Could you sign the CLA for this change, I'll run it on an internal CI branch to merge it.

[dmlambea]

Thanks for this change @dmlambea Could you sign the CLA for this change, I'll run it on an internal CI branch to merge it.

I've done it already. It seem the "CLA not signed yet" comment has been registered after the "CLA signed" one.

[milindl]

Right @dmlambea , I think the CLA signatures got reset for some reason for all the contributors at once. Could you retry signing it once, if that works? Thanks

[dmlambea]

Right @dmlambea , I think the CLA signatures got reset for some reason for all the contributors at once. Could you retry signing it once, if that works? Thanks

I've retried a minute ago with success. cla-assistant.io says: "You have agreed to the CLA for confluentinc/confluent-kafka-go" and below there is the version of the CLA (2023-08-04) along with the complete text and my personal data.

[milindl]

Thank you. I created the internal branch and raised a PR with that so the CI can run. #1077

[milindl]

Merged the internal branch after CI checks.
Thanks for the issue and the PR @dmlambea !