Enhancement - Refactor system tests of environment `enrollment_cluster`

[Rebits]

Description

As specified in https://github.com/wazuh/qa-system-framework/issues/34, it is required to refactor all system tests that belong to the agentless_cluster environment.

List of tests

• [ ] Refactor system test test_agent_enrollment
• [ ] Refactor system test test_agent_default_group_added
• [ ] Refactor system test test_assign_agent_group_with_enrollment
• [ ] Refactor system test test_assign_agent_never_connected_to_group
• [ ] Refactor system test test_assign_agent_to_a_group_api
• [ ] Refactor system test test_assign_agent_to_a_group_by_tool
• [ ] Refactor system test test_assign_groups_guess