Selftests: add html optional plugins tests

To non-parallel execution script.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

selftests/run

@@ -40,7 +40,9 @@ def test_suite():
                ('avocado-framework-plugin-runner-vm',
                 'runner_vm'),
                ('avocado-framework-plugin-varianter-cit',
-                'varianter_cit'))
+                'varianter_cit'),
+               ('avocado-framework-plugin-result-html',
+                'html'))
     for plugin_name, plugin_dir in plugins:
         if plugin_available(plugin_name):
             path = os.path.join(basedir, 'optional_plugins',