chore: Add testing documentation

[JorTurFer]

I have ported the documentation that we provided during graduation DD to the repository in order to make it publicly accessible and easy to find

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Fixes #4173

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[SpiritZhou]

Just want to share something when writing e2e tests. When I try to run my new e2e tests locally, the setup command ("go test -v -tags e2e ./utils/setup_test.go") deployed the official KEDA image, which is not convenient for debugging new tests with a changed KEDA. I suggest adding a parameter in the setup command for deploying the custom image repository so that developers can use their KEDA repository for debugging.

[JorTurFer]

Just want to share something when writing e2e tests. When I try to run my new e2e tests locally, the setup command ("go test -v -tags e2e ./utils/setup_test.go") deployed the official KEDA image, which is not convenient for debugging new tests with a changed KEDA. I suggest adding a parameter in the setup command for deploying the custom image repository so that developers can use their KEDA repository for debugging.

It's already supported but not properly documented I think. As KEDA is deployed using make, and Makefile uses variables, you can set those envs (globally o for the command) and that's all.
Let me update the PR with that info

[JorTurFer]

@SpiritZhou @zroubalik PTAL

[SpiritZhou]

@SpiritZhou @zroubalik PTAL

It looks great now!

[JorTurFer]

/skip-e2e

[JorTurFer]

lol, although I requested a revision of @zroubalik , looks like GH already took into account the previous approval and the automerger did its job. I guess that I had to dismiss the approval instead or request another review :/