responder-get-domains-tests: fix checking of leaks
    
    global_talloc_context should not be NULL in tests. It should be initialised
    with function leak_check_setup otherwise memory leak will not be detected.
    
    check_leaks_pop should not be directly called for global_talloc_context.
    It is a purpose of function leak_check_teardown
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>