swim: be ready to idle round steps when net is slow

First of all, the problem in a nutshell was that ev_timer with
non-zero 'repeat' field in fact is a ev_periodic. It is restarted
*automatically*, even if a user does not write ev_timer_again()
nor ev_timer_start().

This led to a situation, that a round message send is scheduled,
and next round step timer alarm happens before the message is
actually sent. It, in turn, led to an assertion on attempt to
schedule a task twice.

This patch fixes the swim test harness to behave like ev_timer
with 'repeat' > 0, and on first idle round step stops the timer -
it will be restarted once the currently hanging task will be
finally sent.

Follow up #3234