Fix test classes name of test_tokens.py

These test classes 'UsersTestJSON' and 'UsersTestXML' were introduced
for testing tokens in
  Change-Id: I9a810844b1d991f829f411db308e37a02bd6c09e
But their names are not suitable for their responsibility.
This commit fixes it.

Change-Id: I2bb51fe147c17846d667be6f259f63dcaeffe3fc
diff --git a/tempest/api/identity/admin/v3/test_tokens.py b/tempest/api/identity/admin/v3/test_tokens.py
index 802113a..9629213 100644
--- a/tempest/api/identity/admin/v3/test_tokens.py
+++ b/tempest/api/identity/admin/v3/test_tokens.py
@@ -19,7 +19,7 @@
 from tempest.test import attr
 
 
-class UsersTestJSON(base.BaseIdentityV3AdminTest):
+class TokensV3TestJSON(base.BaseIdentityV3AdminTest):
     _interface = 'json'
 
     @attr(type='smoke')
@@ -51,5 +51,5 @@
                           subject_token)
 
 
-class UsersTestXML(UsersTestJSON):
+class TokensV3TestXML(TokensV3TestJSON):
     _interface = 'xml'