tests/user/test_users_present_slice.yml: Fix missing users.json

users.json is generated for the tests and not part of the repo any more.
This test was lacking the include to generate the file.

Related to: b7e1a99b
            tests/user/test_users*.yml: Use extended dynamic users.json

tests/user/test_users_present_slice.yml

 ---
+- name: Include create_users_json.yml
+  import_playbook: create_users_json.yml
+
 - name: Test users present slice
   hosts: ipaserver
   become: true