test: flaky replication/gh-4402-info-errno.test.lua

On heavy loaded hosts found the following issue:

  [004] --- replication/gh-4402-info-errno.result	Wed Jul 22 06:13:34 2020
  [004] +++ replication/gh-4402-info-errno.reject	Wed Jul 22 06:41:14 2020
  [004] @@ -32,7 +32,39 @@
  [004]   | ...
  [004]  d ~= nil and d.status == 'follow' or i
  [004]   | ---
  [004] - | - true
  [004] + | - version: 2.6.0-10-g8df49e4
  [004] + |   id: 1
  [004] + |   ro: false
  [004] + |   uuid: 41c4e3bf-cc3b-443d-88c9-39a9a8fe2df9
  [004] + |   package: Tarantool
  [004] + |   cluster:
  [004] + |     uuid: 6ec7bcce-68e7-41a4-b84b-dc9236621579
  [004] + |   listen: unix/:(socket)
  [004] + |   replication_anon:
  [004] + |     count: 0
  [004] + |   replication:
  [004] + |     1:
  [004] + |       id: 1
  [004] + |       uuid: 41c4e3bf-cc3b-443d-88c9-39a9a8fe2df9
  [004] + |       lsn: 52
  [004] + |     2:
  [004] + |       id: 2
  [004] + |       uuid: 8a989231-177a-4eb8-8030-c148bc752b0e
  [004] + |       lsn: 0
  [004] + |       downstream:
  [004] + |         status: stopped
  [004] + |         message: timed out
  [004] + |         system_message: Connection timed out
  [004] + |   signature: 52
  [004] + |   status: running
  [004] + |   vclock: {1: 52}
  [004] + |   uptime: 27
  [004] + |   lsn: 52
  [004] + |   sql: []
  [004] + |   gc: []
  [004] + |   vinyl: []
  [004] + |   memory: []
  [004] + |   pid: 99
  [004]   | ...
  [004]
  [004]  test_run:cmd('stop server replica')

It happened because replication downstream status check occurred too
early, when it was only in 'stopped' state. To give the replication
status check routine ability to reach the needed 'follow' state, it
need to wait for it using test_run:wait_downstream() routine.

Closes #5235