TestThemeExtensionList example

'theme' => [],
        'disabled' => [
          'theme' => [],
        ],
        'theme_engine' => '',
      ],
    ]);

    $theme_engine_list = new TestThemeEngineExtensionList($this->root, 'theme_engine', new NullBackend('test'), $info_parser->reveal(), $module_handler->reveal(), $state, $config_factory);
    $theme_engine_list->setExtensionDiscovery($extension_discovery->reveal());

    $theme_list = new TestThemeExtensionList($this->root, 'theme', new NullBackend('test'), $info_parser->reveal(), $module_handler->reveal(), $state, $config_factory, $theme_engine_list, 'testing');
    $theme_list->setExtensionDiscovery($extension_discovery->reveal());

    $theme_data = $theme_list->reset()->getList();
    $this->assertCount(2, $theme_data);

    $info_basetheme = $theme_data['test_basetheme'];
    $info_subtheme = $theme_data['test_subtheme'];

    // Ensure some basic properties.     $this->assertInstanceOf('Drupal\Core\Extension\Extension', $info_basetheme);
    $this->assertEquals('test_basetheme', $info_basetheme->getName());