Merge volume message list and show testcases

test_list_messages and test_show_message create a volume
type and a volume respectively which is not necessary,
in other words, they can share a volume type and volume
and message, so we can spare one create_volume_type and
one create_volume.

We can either create a volume type and a volume in
resource_setup for sharing usage, or to merge test_list_messages
and test_show_message, but since UserMessagesTest uses
min_microversion which can only have effect after setUp,
that means we can't call list_messages in resource_setup,
so this is to merge list and show into one testcase.

Change-Id: I68fa98697dfb74ec16e4bdb970bad6737817e70b
diff --git a/tempest/api/volume/admin/test_user_messages.py b/tempest/api/volume/admin/test_user_messages.py
index 20c3538..9907497 100644
--- a/tempest/api/volume/admin/test_user_messages.py
+++ b/tempest/api/volume/admin/test_user_messages.py
@@ -62,8 +62,16 @@
         return message_id
 
     @decorators.idempotent_id('50f29e6e-f363-42e1-8ad1-f67ae7fd4d5a')
-    def test_list_messages(self):
-        self._create_user_message()
+    def test_list_show_messages(self):
+        message_id = self._create_user_message()
+        self.addCleanup(self.messages_client.delete_message, message_id)
+
+        # show message
+        message = self.messages_client.show_message(message_id)['message']
+        for key in MESSAGE_KEYS:
+            self.assertIn(key, message.keys(), 'Missing expected key %s' % key)
+
+        # list messages
         messages = self.messages_client.list_messages()['messages']
         self.assertIsInstance(messages, list)
         for message in messages:
@@ -71,16 +79,6 @@
                 self.assertIn(key, message.keys(),
                               'Missing expected key %s' % key)
 
-    @decorators.idempotent_id('55a4a61e-c7b2-4ba0-a05d-b914bdef3070')
-    def test_show_message(self):
-        message_id = self._create_user_message()
-        self.addCleanup(self.messages_client.delete_message, message_id)
-
-        message = self.messages_client.show_message(message_id)['message']
-
-        for key in MESSAGE_KEYS:
-            self.assertIn(key, message.keys(), 'Missing expected key %s' % key)
-
     @decorators.idempotent_id('c6eb6901-cdcc-490f-b735-4fe251842aed')
     def test_delete_message(self):
         message_id = self._create_user_message()