Add tests for Engine.delete_doc()

Engine.delete_doc() is only tested at the "action" level in functional/test_engine.py. But while testing 4.5.1 and handling that case, I got a bloody TypeError that our CI must have catched:

• from a local sync folder, move out a subfolder
• Drive should ask if I want to cancel or unsync the document
• clinking on cancel will restore the document (but in my case, I had the exception)

The error was introduced with NXDRIVE-1768:

TypeError: scan_remote() takes 1 positional argument but 2 were given
  File "nxdrive/gui/application.py", line 715, in _doc_deleted
  File "nxdrive/engine/engine.py", line 591, in rollback_delete
  File "nxdrive/engine/activity.py", line 263, in func_wrapper

Several tests must be added to cover all possible solutions the user can choose and use the actual code, no mocks.