Replaced file-based config creation with direct struct population.
This eliminates temporary files and makes the test cleaner.
Key changes:
- Added helper functions for creating config structures
- Create CFG_SERVER, CFG_CLIENT, CFG_CLIENT_LINK programmatically
- Use utun_instance_create_from_config() instead of file-based creation
- No temp directories or file cleanup needed
The test now builds configs in memory using helper functions like
make_sockaddr(), create_server(), create_client(), etc.
Evgeny