Write a test to ensure `SkipDir` is supported.

[matryer]

From the Go docs:

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If there was a problem walking to the file or directory named by path, the incoming error will describe the problem and the function can decide how to handle that error (and Walk will not descend into that directory). If an error is returned, processing stops. The sole exception is that if path is a directory and the function returns the special value SkipDir, the contents of the directory are skipped and processing continues as usual on the next file.

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Write a test to ensure SkipDir is supported.

[matryer]

NOTE: this is not supported.