Fix Event test due to a missing plugin [Constraint] (#462)

Author: fredroy

bindings/Sofa/tests/Core/Events.py

@@ -40,6 +40,7 @@ def test_events(self):
         """Test the BuildConstraintSystem and SolveConstraintSystem events."""
         node = Sofa.Core.Node("root")
         node.addObject("RequiredPlugin", name="Sofa.Component.AnimationLoop")
+        node.addObject("RequiredPlugin", name="Sofa.Component.Constraint.Lagrangian.Solver")
         node.addObject("FreeMotionAnimationLoop", name="loop")
         node.addObject("GenericConstraintSolver", name="constraintSolver")
         controller = node.addObject( MyController() )