tests(helpers): split constants into a standalone file

[chronolaw]

Summary

KAG-5233

It is one of serial refactors of helpers.lua

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix #[issue number]

[team-gateway-bot]

Cherry-pick failed for master, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git remote add upstream https://github.com/kong/kong-ee
git fetch upstream master
git worktree add -d .worktree/cherry-pick-13539-to-master-to-upstream upstream/master
cd .worktree/cherry-pick-13539-to-master-to-upstream
git checkout -b cherry-pick-13539-to-master-to-upstream
ancref=$(git merge-base 38727ba119932bd3f040383a2ae8483d5b1b4257 2e4d22b438b5b7304651b55f4531574c86340072)
git cherry-pick -x $ancref..2e4d22b438b5b7304651b55f4531574c86340072

[team-gateway-bot]

Cherry-pick failed for master, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git remote add upstream https://github.com/kong/kong-ee
git fetch upstream master
git worktree add -d .worktree/cherry-pick-13539-to-master-to-upstream upstream/master
cd .worktree/cherry-pick-13539-to-master-to-upstream
git checkout -b cherry-pick-13539-to-master-to-upstream
ancref=$(git merge-base 38727ba119932bd3f040383a2ae8483d5b1b4257 2e4d22b438b5b7304651b55f4531574c86340072)
git cherry-pick -x $ancref..2e4d22b438b5b7304651b55f4531574c86340072