fileloader_test: Replace lengthy t.Fatalf with require

[annasong20]

Replace condition and t.Fatalf() combination with more succinct require() in fileloader tests.

[k8s-ci-robot]

Hi @annasong20. Thanks for your PR.

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[natasha41575]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: annasong20, natasha41575

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