feat: add receive_events_non_blocking

[thalesmg]

Sometimes, we want just to collect any events between two points in time, without waiting for any particular number of events or timeout.

[ieQu1]

Philosophy behind snabbkaffe is to replace all time relationships in tests with causality relationships. Since causality relationships should be knowable, this should make tests more deterministic. "Collecting all events so far" API is obviously nondeterministic and non-causal. Therefore it should not be there.

P.S. There are a lot of old APIs that violate this rule as well (any API that accepts a timeout...), and they should be gradually removed as well. Ideally, the only API that can behave nondeterministically should be the high level timetrap of check_trace macro.