Add tests for diff test-only split helpers

[magic-peach]

What this PR does:

Adds unit tests for the helper functions behind the --split-test-only flag on depstat diff: filterDepsByTestStatus, filterEdgesByTestStatus, filterVersionChangesByTestStatus, computeFilteredCounts, buildSplitSection, buildSplitResult, diffSlices, and getEdges. These functions had no direct unit test coverage even though the flag itself is exercised indirectly elsewhere.

Test only change, no production code touched.

How it was tested:

Ran go test ./... on a clean test cache, full suite passes.

No existing PR or issue covers this. Not linked to an open issue.

[kubernetes-prow]

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[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: magic-peach
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[skitt]

See https://kubernetes.slack.com/archives/C01672LSZL0/p1789992857325439

/close

[kubernetes-prow]

@skitt: Closed this PR.

Details

In response to this:

See https://kubernetes.slack.com/archives/C01672LSZL0/p1789992857325439

/close

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