On heavy loaded hosts found the following 3 issues:

line 174:

 [026] --- replication/status.result	Thu Jun 11 12:07:39 2020
 [026] +++ replication/status.reject	Sun Jun 14 03:20:21 2020
 [026] @@ -174,15 +174,17 @@
 [026]  ...
 [026]  replica.downstream.status == 'follow'
 [026]  ---
 [026] -- true
 [026] +- false
 [026]  ...

It happened because replication downstream status check occurred too
early. 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.

line 178:

[024] --- replication/status.result	Mon Sep  7 00:22:52 2020
[024] +++ replication/status.reject	Mon Sep  7 00:36:01 2020
[024] @@ -178,11 +178,13 @@
[024]  ...
[024]  replica.downstream.vclock[master_id] == box.info.vclock[master_id]
[024]  ---
[024] -- true
[024] +- error: '[string "return replica.downstream.vclock[master_id] =..."]:1: attempt to
[024] +    index field ''vclock'' (a nil value)'
[024]  ...
[024]  replica.downstream.vclock[replica_id] == box.info.vclock[replica_id]
[024]  ---
[024] -- true
[024] +- error: '[string "return replica.downstream.vclock[replica_id] ..."]:1: attempt to
[024] +    index field ''vclock'' (a nil value)'
[024]  ...
[024]  --
[024]  -- Replica

It happened because replication vclock field was not exist at the moment
of its check. To fix the issue, vclock field had to be waited to be
available using test_run:wait_cond() routine. Also the replication data
downstream had to be read at the same moment.

line 224:

[014] --- replication/status.result	Fri Jul  3 04:29:56 2020
[014] +++ replication/status.reject	Mon Sep  7 00:17:30 2020
[014] @@ -224,7 +224,7 @@
[014]  ...
[014]  master.upstream.status == "follow"
[014]  ---
[014] -- true
[014] +- false
[014]  ...
[014]  master.upstream.lag < 1
[014]  ---

It happened because replication upstream status check occurred too
early. To give the replication status check routine ability to reach
the needed 'follow' state, it need to wait for it using
test_run:wait_upstream() routine.

Removed test from 'fragile' test_run tool list to run it in parallel.

Closes #5110