TESTS: Add a test for parallel execution of klist
    
    Integration test for:
        https://pagure.io/SSSD/sssd/issue/3372
    
    With https://pagure.io/SSSD/sssd/issue/3372 still broken, the unit test
    wold fail because one of the concurrent klist commands would trigger a
    race condition in the KCM queue code, crashing the KCM responder.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>