Check idempotence in tests

[shadr]

This PR adds an idempotence check in tests.
To make tests pass with the idempotence check I made two fixes:

1. Fix the case when extends statement was stealing docstring from the next statement

   extends Node
   
   ## docstring
   func foo():
       pass

   previously formatted as

   extends Node
   ## docstring
   
   func foo():
       pass

   and when there is no blank line around docstring attach it to the next statement instead of the extends statement:

   extends Node
   ## docstring
   func foo():
       pass

   previously formatted as

   extends Node
   ## docstring
   
   func foo():
       pass

   Now in both cases it attaches docstring to the statemet

2. When reordering attach docstring to the class only if it does not attached to the statement
   Previously code with --reorder-code

   extends Node
   
   ## docstring
   func foo():
       pass

   got transformed into

   extends Node
   ## docstring
   
   
   func foo():
       pass

   Now it correctly attaches docstring to the function definition.

   I'm not that familiar with the reordering code, so I hope I didn't break anything. I also left the TODO intact because I have a feeling I didn't cover all the cases.

[NathanLovato]

Great, thank you! Just added a message for context that will tell us if it's the first formatting pass or idempotence check that failed.