Found that on heavy loaded hosts the test tries to check replication
downstream status when downstream structure is not ready and it fails
with the error:

[017] --- replication/wal_rw_stress.result	Thu Jul  9 17:04:16 2020
[017] +++ replication/wal_rw_stress.reject	Fri May  8 08:25:15 2020
[017] @@ -75,7 +75,8 @@
[017]      return box.info.replication[1].downstream.status ~= 'stopped' \
[017]  end) or box.info
[017]  ---
[017] -- true
[017] +- error: '[string "return test_run:wait_cond(function()         ..."]:1: attempt to
[017] +    index field ''downstream'' (a nil value)'
[017]  ...
[017]  test_run:cmd("switch default")
[017]  ---
[017]

So the wait condition should start from the check of the downstream
structure availability.

Follows up #4977