New test for active_later->active transition on event_active

When event_active is called, an active_later event is supposed to become
active now.  We had lacked a test for that.
New tests for event_base_foreach_event()

Our dump-events code had exercised this a bit, but only via the
nonlocking backend. Also, nothing was checking the "Search for a
specific event" code.
Unit tests for event_base_gettimeofday_cached() and event_base_update_cache_time()
A test for event_get_assignment()