test: flaky replication/gh-3704-misc-*

On heavy loaded hosts found the following issue:

  [037] --- replication/gh-3704-misc-replica-checks-cluster-id.result	Thu Sep 10 18:05:22 2020
  [037] +++ replication/gh-3704-misc-replica-checks-cluster-id.reject	Fri Sep 11 11:09:38 2020
  [037] @@ -25,7 +25,7 @@
  [037]  ...
  [037]  box.info.replication[2].downstream.status
  [037]  ---
  [037] -- follow
  [037] +- stopped
  [037]  ...
  [037]  -- change master's cluster uuid and check that replica doesn't connect.
  [037]  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 #5293

test/replication/gh-3704-misc-replica-checks-cluster-id.result

@@ -23,9 +23,9 @@ test_run:grep_log("replica", "REPLICASET_UUID_MISMATCH")
 ---
 - null
 ...
-box.info.replication[2].downstream.status
+test_run:wait_downstream(2, {status = 'follow'})
 ---
-- follow
+- true
 ...
 -- change master's cluster uuid and check that replica doesn't connect.
 test_run:cmd("stop server replica")

test/replication/gh-3704-misc-replica-checks-cluster-id.test.lua

@@ -9,7 +9,7 @@ test_run:cmd("create server replica with rpl_master=default, script='replication
 box.schema.user.grant("guest", "replication")
 test_run:cmd("start server replica")
 test_run:grep_log("replica", "REPLICASET_UUID_MISMATCH")
-box.info.replication[2].downstream.status
+test_run:wait_downstream(2, {status = 'follow'})
 -- change master's cluster uuid and check that replica doesn't connect.
 test_run:cmd("stop server replica")
 _ = box.space._schema:replace{'cluster', tostring(uuid.new())}