On heavy loaded hosts either slow machines like VMware found the
following issues:

  [010] --- replication/long_row_timeout.result   Fri May  8 08:56:08 2020
  [010] +++ var/rejects/replication/long_row_timeout.reject       Mon Jun 21 04:39:08 2021
  [010] @@ -23,7 +23,7 @@
  [010]  ...
  [010]  box.info.replication[2].downstream.status
  [010]  ---
  [010] -- follow
  [010] +- stopped
  [010]  ...
  [010]  -- make applier incapable of reading rows in one go, so that it
  [010]  -- yields a couple of times.
  [010]

It happened because replication downstream status check occurred too
early, when it was only in 'stopped' state. This situation happens
when replica done its initial join, but not reached subscription yet.
To give the replication status check routine ability to reach the
needed 'follow' state, it need for it using test_run:wait_downstream()
routine.

We don't see this issue anymore, but let's fix it still in case we ever
encounter it again.

Also remove the test from fragile list.

Closes #4351

Co-developed-by: Serge Petrenko <sergepetrenko@tarantool.org>