Modify scale down set processor to add reasons to unremovable nodes

[abdelrahman882]

What type of PR is this?

/kind bug
/kind cleanup

What this PR does / why we need it:

Modify scale down set processor to add reasons to unremovable nodes so that planner can have these nodes excluded as unremovable nodes with reasons

Special notes for your reviewer:

1. Moved NewTestProcessors to be under processors/test to avoid the following circular dependency : nodes (ScaleDownSetProcessor)-> ScaleDownContext-> resource -> test (core/test/common)-> nodes (ScaleDownSetProcessor)
2. On adding unit tests for planner I noticed that MaxNodesProcessor is being called before AtomicResizeFilteringProcessor which might cause filtering removable nodes on both calls ending up with less nodes to be deleted than we should be deleting, created an issue for it cluster-autoscaler : MaxNodesProcessor is being called before AtomicResizeFilteringProcessor #7342

Does this PR introduce a user-facing change?

NONE

[k8s-ci-robot]

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[bskiba]

Thank you for the change! Especially for adding unit tests where they were missing.

I've got some suggestions but the comments are all relatively minor.

[bskiba]

/lgtm

[bskiba]

I think you can remove the isValidCandidatesFiltering function now. Otherwise
/lgtm

[bskiba]

/lgtm

[x13n]

Overall looks good, just minor comments.

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: abdelrahman882, bskiba, x13n

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[x13n]

/lgtm

[SarCode]

@abdelrahman882 this is such an amazing change, and I was about to put this out as a feature request 🙌🏻

Unfortunately I don't see the annotation part (where it is supposed to mention on the node about blockers) present in any of the releases done after this merge, is there any blocker that I can help with? @x13n @omerap12

Edit: Seems like I misunderstood the code. It seems to be only giving this info internally and not storing it in CM or annotating the node