ensure setup_test_user has been called before using test_user

when tempest run in one process, setup_test_user can be called in
other tests. But when tempest run in multiple process, we should
ensure it has been called before using test_user in every test.

Part of blueprint speed-up-tempest

Change-Id: If5aadf0852c578fb198f49be3604d6cbe24155f9
diff --git a/tempest/tests/identity/admin/test_users.py b/tempest/tests/identity/admin/test_users.py
index 27a214c..1423741 100644
--- a/tempest/tests/identity/admin/test_users.py
+++ b/tempest/tests/identity/admin/test_users.py
@@ -195,6 +195,7 @@
     @attr(type='negative')
     def test_authentication_with_invalid_username(self):
         # Non-existent user's token should not get authenticated
+        self.data.setup_test_user()
         self.assertRaises(exceptions.Unauthorized, self.token_client.auth,
                           'junkuser123', self.data.test_password,
                           self.data.test_tenant)