fix: re-enable gcs tests in pipeline

[robklg]

This PR re-enables the GCS tests. By pinning to an older version of the fsouza/fake-gcs-server, which stopped supporting files ending with / in v1.43.0

We should replace this docker-dependent setup with something more robust.

Also reintroducing root_dir setting, which serves as a regression test for https://github.com/bolcom/libunftp/issues/508