[executorch][flat_tensor] update flat tensor header test

[pytorchbot]

This PR was created by the merge bot to help merge the original PR into the main branch.
ghstack PR number: #8109
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ghstack PR base: https://github.com/pytorch/executorch/tree/gh/lucylq/33/base
ghstack PR head: https://github.com/pytorch/executorch/tree/gh/lucylq/33/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/main
Merge bot PR head: https://github.com/pytorch/executorch/tree/gh/lucylq/33/orig
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[pytorch-bot]

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