Fixed TestAssetsFromDir unit test failure on Windows

[bobsira]

• Added a t.Cleanup hook that iterates over every CopyableFile returned by assetsFromDir and calls Close(). This releases the open file descriptors created by assets.NewFileAsset.
• This directly fixes the Windows test failure where RemoveAll couldn’t delete temp files because they were still open.
• Switched some non-fatal t.Errorf + return cases to t.Fatalf for earlier, clearer abort when setup fails (creating test files or gathering assets).

[k8s-ci-robot]

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[k8s-ci-robot]

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