Enable nested group test only when nested groups are supported

spec/models/ee/namespace_spec.rb

@@ -57,6 +57,7 @@ describe Namespace, models: true do
         end
       end
 
+      if Group.supports_nested_groups?
         context 'when license is applied to parent group' do
           let(:child_group) { create :group, parent: group }
 
@@ -65,6 +66,7 @@ describe Namespace, models: true do
           end
         end
       end
+    end
 
     context 'when feature not available' do
       let(:feature) { :deploy_board }