Reorganize test helper packages

[pmalek]

Problem statement

Currently, there exist many test helper/util/(?) packages e.g.

• https://github.com/Kong/kubernetes-ingress-controller/tree/main/test/helpers
• https://github.com/Kong/kubernetes-ingress-controller/blob/main/test/internal
• https://github.com/Kong/kubernetes-ingress-controller/blob/5a52b842e01c0462c5a5421b96671faf065cd5d4/test/tcp_utils.go
• https://github.com/Kong/kubernetes-ingress-controller/tree/main/internal/util/test

This issue tracks the reorganization of those into 1 or 2 places and ideally documenting the final result.

It might not be easy to put those in 1 package because we're using (rightly so) internal packages and some functions/consts are used cross top level packages (e.g. some are defined in top level test/internal/ but could be used in unit tests etc.

Acceptance criteria

• [ ] Test helpers functions are placed in 1 (or 2) packages
• [ ] The above is documented in TESTING.md

Originally posted by @pmalek in https://github.com/Kong/kubernetes-ingress-controller/pull/4823#discussion_r1394491625