osmo-gsm-tester: Convert event_loop.wait to support passive polling
Right now, all polling in osmo-gsm-tester is done by sleeping small amounts of times (like 0.1 seconds), then calling poll function everywhere. Some of those poll functions are actually event-based (like glib main loop, or any fd/socket being used) and can be monitored with select() instead of using active polling. This way we avoid wasting resources and we avoid delaying signal-driven events.
For objects not supporting passive polling, we should provide an API to create a timer object (man timerd_create ?) and just call select() on those.