Bugfix: Reproducible layers with whiteout

[hlubek]

Fixes #900 (which was not fully solved).

Description

When deleting files of previous layers, the whiteout files
were not added to the tar file in a consistent order.

This change adds a stable sorting to the whiteout files and
adds unit tests to check for stable sorting.

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• Includes unit tests
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Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

No user facing changes.

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[hlubek]

The code is not ideal (using defer in a loop) and the non-determinism is a little hard to understand (converting from a map to a slice). But I chose to impolement the new tests like TestSnasphotPreservesFileOrder for consistency.

[hlubek]

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