Fixes a test case name.

Most tempest testcase name ends with "XML" or "JSON", this fix just make
one testcase keep consistent with the others.

Change-Id: I5fd0f16f34ef46db675ddcf99d3723818859ce7f
diff --git a/tempest/api/compute/servers/test_servers_negative.py b/tempest/api/compute/servers/test_servers_negative.py
index 5f53080..5cc8dc6 100644
--- a/tempest/api/compute/servers/test_servers_negative.py
+++ b/tempest/api/compute/servers/test_servers_negative.py
@@ -24,12 +24,12 @@
 from tempest.test import attr
 
 
-class ServersNegativeTest(base.BaseComputeTest):
+class ServersNegativeTestJSON(base.BaseComputeTest):
     _interface = 'json'
 
     @classmethod
     def setUpClass(cls):
-        super(ServersNegativeTest, cls).setUpClass()
+        super(ServersNegativeTestJSON, cls).setUpClass()
         cls.client = cls.servers_client
         cls.img_client = cls.images_client
         cls.alt_os = clients.AltManager()
@@ -248,5 +248,5 @@
                           '999erra43')
 
 
-class ServersNegativeTestXML(ServersNegativeTest):
+class ServersNegativeTestXML(ServersNegativeTestJSON):
     _interface = 'xml'