ValueTreePropertyWithDefault: Initialise MessageManager in default setting unit test

modules/juce_data_structures/values/juce_ValueTreePropertyWithDefault_test.cpp

@@ -64,6 +64,8 @@ public:
 
         beginTest ("set default");
         {
+            ScopedJuceInitialiser_GUI scopedJuceInitialiser_gui;
+
             ValueTree t ("root");
 
             ValueTreePropertyWithDefault value (t, "testkey", nullptr);