Add unit tests for sortedDomainsWithLeader sorting behavior

[IrvingMg]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Adds unit tests for sortedDomainsWithLeader to document and verify its sorting behavior.

Which issue(s) this PR fixes:

Closes #8178

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

[netlify]

✅ Deploy Preview for kubernetes-sigs-kueue ready!

Name	Link
🔨 Latest commit	5db2e50
🔍 Latest deploy log	https://app.netlify.com/projects/kubernetes-sigs-kueue/deploys/693bf8e50ffe5a00088b001b
😎 Deploy Preview	https://deploy-preview-8207--kubernetes-sigs-kueue.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: IrvingMg
Once this PR has been reviewed and has the lgtm label, please assign tenzen-y for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[IrvingMg]

The current behavior seems to be correct. In the sortedDomainsWithLeader function, only sliceStateWithLeader changes depending on the algorithm. stateWithLeader is only used to break ties and is always sorted in ascending order. This matches how sortedDomains works, where the state is always sorted ascending to help prioritize domains that can fit slices with minimal leftover capacity.

kueue/pkg/cache/scheduler/tas_flavor_snapshot.go

Lines 1326 to 1333 in 98e0af8

	// This function sorts domains based on a specified algorithm: BestFit or LeastFreeCapacity.
	//
	// The sorting criteria are:
	// - **BestFit**: `sliceState` (descending), `state` (ascending), `levelValues` (ascending)
	// - **LeastFreeCapacity**: `sliceState` (ascending), `state` (ascending), `levelValues` (ascending)
	//
	// `state` is always sorted ascending. This prioritizes domains that can accommodate slices with minimal leftover pod capacity.
	func (s *TASFlavorSnapshot) sortedDomains(domains []*domain, unconstrained bool) []*domain {

/cc @PBundyra @mimowo