Fix overriding of filesystem info
Previously, `BaseCompressedFileFileSystem.info` was overridden and transformed from function to dict.

This generated a bug for filesystem methods that use `self.info()`, like e.g. `fs.isfile()`.

This PR:
- Adds tests for `fs.isfile` (that use `fs.info`).
- Fixes custom `BaseCompressedFileFileSystem.info` by removing its overriding.