old_contents: import unittest
  from stevedore.extension import ExtensionManager
  
  PLUGINS = [
      'mayavi_tools']
  
  
  class TestVisualisationPluginImport(unittest.TestCase):
  
      def test_plugin_import(self):
          extension_manager = ExtensionManager(namespace='simphony.visualisation')
          for engine in PLUGINS:
              if engine not in extension_manager:
                  self.fail("`{}` could not be imported".format(engine))
  
new_contents: import unittest
  from stevedore.extension import ExtensionManager
  
  PLUGINS = [
      'mayavi_tools',
      'aviz']
  
  
  class TestVisualisationPluginImport(unittest.TestCase):
  
      def test_plugin_import(self):
          extension_manager = ExtensionManager(namespace='simphony.visualisation')
          for engine in PLUGINS:
              if engine not in extension_manager:
                  self.fail("`{}` could not be imported".format(engine))
  
subject: add aviz in the test_visualisation_import