test: drop invalid assert from swim test transport

The assertion was checking that a next event object is not the
same as the previous, but

    1) the previous was deleted already to this moment;
    2) comparison was done by pointer

The first problem would be enough to drop it. The second is
already curious - looks like after the old event was deleted,
the next event was allocated right on the same memory. This is
why their pointers are equal and the assertion fails.

For example, swim_timer_event_process() - it deletes the event
object and calls ev_invoke() which can generate a new event on
the just freed memory.