Skip test_extractor only for zstd param if zstandard not installed
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`,...).
_The documentation is not available anymore as the PR was closed or merged._