Skip test_extractor only for zstd param if zstandard not installed
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Currently, if `zstandard` is not installed, `test_extractor` is skipped for all compression format parameters.

This PR fixes `test_extractor` so that if `zstandard` is not installed, `test_extractor` is skipped only for the `zstd` compression parameter, that is, it is not skipped for all the other compression parameters (`gzip`, `xz`,...).