Refactor `basic_cluster` system tests

[juliamagan]

Description

When basic_cluster tests were launched together, some tests failed. Upon investigation it was found that these tests were environment dependent and that the test_agent_groups_forced_change.py test changed the environment, causing the rest to fail on the following executions. Also, some tests are old and could be improved. The tests to be refactored are the following:

• tests/system/test_cluster/test_agent_files_deletion/test_agent_files_deletion.py
• tests/system/test_cluster/test_agent_groups/test_agent_groups_forced_change.py
• tests/system/test_cluster/test_agent_info_sync/test_agent_info_sync.py
• tests/system/test_cluster/test_agent_key_polling/test_agent_key_polling.py
• tests/system/test_multigroups/test_multigroups.py