Maybe....

message.module

@@ -21,6 +21,7 @@ define('MESSAGE_PURGE_LIMIT', 100);
 function message_ctools_plugin_type() {
   $plugins['message_arguments'] = array(
     'classes' => array('class'),
+    'child plugins' => TRUE,
   );
 
   return $plugins;

message.test

@@ -24,7 +24,7 @@ class MessageArgumentsTestCase extends DrupalWebTestCase {
   }
 
   function setUp() {
-    parent::setUp(array('message'));
+    parent::setUp('message', 'message_example');
 
     $this->user = $this->drupalCreateUser();
   }
@@ -33,24 +33,16 @@ class MessageArgumentsTestCase extends DrupalWebTestCase {
    * Testing ctools message arguments plugin.
    */
   public function testCtoolsArguments() {
-    module_enable(array('message_example'));
-    $query = new EntityFieldQuery();
-    $reqults = $query
-      ->entityCondition('entity_type', 'message_type')
-      ->execute();
-
-    if ($reqults['message_type']) {
-      $foo = entity_load('message_type', array_keys($reqults['message_type']));
-      debug($foo);
-    }
-
-
     $uri = entity_uri('user', $this->user);
 
     $message = message_create('example_arguments', array('uid' => $this->user->uid));
     $message->save();
 
-    $arguments = message_get_message_arguments_handler($message)->getArguments();
+    if (!$handler = message_get_message_arguments_handler($message)) {
+      throw new Exception('No arguments handler was found for the Message example message type.');
+    }
+
+    $arguments = $handler->getArguments();
 
     $expected_arguments = array(
       '@name' => $this->user->name,