test: replication/misc fix for high load

Fixed test replication/misc to be able to run it on
hosts under high load. Changed downstream check to
use test_run:wait_downstream() function, to wait for
certain box.info.replication values to fix the error:

[050] replication/misc.test.lua                                       [ fail ]
[050]
[050] Test failed! Result content mismatch:
[050] --- replication/misc.result	Thu Jun  6 06:46:54 2019
[050] +++ replication/misc.reject	Fri Jun  7 05:55:00 2019
[050] @@ -622,7 +622,7 @@
[050]  ...
[050]  box.info.replication[2].downstream.status
[050]  ---
[050] -- stopped
[050] +- follow
[050]  ...
[050]  test_run:cmd("stop server replica")
[050]  ---

Closes #4277