Drive: add some tests on concurrency issues

For instance:

• Having 2 clients bound to the same server with the same user account, deleting a synchronized folder on one of the devices should delete the folder on the server and the other device.
• Locally creating a folder, making a synchronization iteration (implying its remote creation) then locally deleting the folder and making another synchronization iteration (equivalent to delaying the fetch of remote changes) should result in deletion of the remote folder.
  Indeed: even though fetching the remote changes will send 'documentCreated' events for the test folder and its content as a result of the first synchronization iteration, since the folder will not have been modified since last synchronization, its pair state will not be marked as 'modified', see Model.update_remote(). Thus the pair state will be ('deleted', 'synchronized'), resolved as 'locally_deleted'.