tests: Remove an unused variable

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
mongo-sync: New function: mongo_sync_cmd_get_last_error_full()

This implements mongo_sync_cmd_get_last_error_full(), which is like
mongo_sync_cmd_get_last_error(), except it returns the full BSON object,
not just the err string. Fixes #30.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>