test changes:
- added cb tests for global params: echo-client-id, decline-probation-period, next-server, server-hostname, boot-file-name
- added cb test for modifying subnet
- added test for enabled echo-client-id
common improvements:
- setup_server_for_config_backend_cmds function can set some initial config params
- added function for sending DECLINE
- set_subnet can set any subnet param now
- set_network can set any network and its subnet param now
- set_global_parameter can sey any global param now
- get_address can check received client-id, next-server, server-hostname and boot-file-name