[Test] Fix test_dump_json test (sofa-framework#488)

* load statecontainer to get MechanicalObject in the factory

* fix missing plugin test (grid)

Author: fredroy

bindings/Sofa/tests/Core/ForceField.py

@@ -108,6 +108,7 @@ def simulate_beam(linear_solver_template):
 
         root.addObject('DefaultAnimationLoop')
 
+        root.addObject('RequiredPlugin', name='Sofa.Component.Topology.Container.Grid')
         root.addObject('RequiredPlugin', name='Sofa.Component.ODESolver.Backward')
         root.addObject('RequiredPlugin', name='Sofa.Component.LinearSolver.Direct')
         root.addObject('RequiredPlugin', name='Sofa.Component.Engine.Select')

bindings/Sofa/tests/Core/ObjectFactory.py

@@ -1,9 +1,10 @@
 import unittest
 import Sofa
+import SofaRuntime
 
 class Test(unittest.TestCase):
 
     def test_dump_json(self):
-
+        SofaRuntime.importPlugin("Sofa.Component.StateContainer")
         json = Sofa.Core.ObjectFactory.dump_json()
         self.assertIn("MechanicalObject", json, "MechanicalObject is not found in the json dump of the ObjectFactory")