Rename test/filestore to test/objectstore

Now ObjectStore support three backend types, so we need to make each backend
share unit test to avoid duplicate codes.

This patch mainly make workload_generator workable for objectstore.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Merge pull request #1168 from yuyuyu101/wip-refactor-objectstore-test

Rename test/filestore to test/objectstore

Reviewed-by: Sage Weil <sage@inktank.com>