[TEST] Convert expectFont function to jest extension

[csouchet]

Covers #673

[tbouffard]

LGTM
New failure message is great

Expected font of the cell with id 'start_event_none_id' to be equals to:
  {"fontFamily": "Nirvana", "fontSize": 11.5, "fontStyle": 1}
Received:
  {"fontFamily": "Arial", "fontSize": 11, "fontStyle": 1}

Difference:

- Expected
+ Received

  Object {
-   "fontFamily": "Nirvana",
-   "fontSize": 11.5,
+   "fontFamily": "Arial",
+   "fontSize": 11,
    "fontStyle": 1,
  }