test: fix flaky replication/wal_off.test.lua

Found issue:

[003] --- replication/wal_off.result	Thu Apr 25 13:10:18 2019
[003] +++ replication/wal_off.reject	Tue Jul 16 17:10:31 2019
[003] @@ -95,6 +95,8 @@
[003]  ...
[003]  while string.find(box.info.replication[wal_off_id].upstream.message, check) == nil do fiber.sleep(0.01) end
[003]  ---
[003] +- error: '[string "while string.find(box.info.replication[wal_of..."]:1: bad argument
[003] +    #1 to ''find'' (string expected, got nil)'
[003]  ...
[003]  box.cfg { replication = "" }
[003]  ---

To check the upstream status and it's message need to wait until an
upstream appears. This prevents an attempt to index a nil value when
one of those functions are called before a record about a peer appears
in box.info.replication. It was observed on test:
  replication/show_error_on_disconnect
after commit
  c6bea65 ('replication: recfg with 0
quorum returns immediately').

Closes #4355

test/replication/wal_off.result

@@ -90,11 +90,9 @@ box.cfg { replication = wal_off_uri }
 box.cfg { replication_sync_timeout = replication_sync_timeout }
 ---
 ...
-check = "Read access to universe"
----
-...
-while string.find(box.info.replication[wal_off_id].upstream.message, check) == nil do fiber.sleep(0.01) end
+test_run:wait_upstream(wal_off_id, {status = 'loading', message_re = "Read access to universe"})
 ---
+- true
 ...
 box.cfg { replication = "" }
 ---

test/replication/wal_off.test.lua

@@ -31,8 +31,7 @@ replication_sync_timeout = box.cfg.replication_sync_timeout
 box.cfg { replication_sync_timeout = 0.01 }
 box.cfg { replication = wal_off_uri }
 box.cfg { replication_sync_timeout = replication_sync_timeout }
-check = "Read access to universe"
-while string.find(box.info.replication[wal_off_id].upstream.message, check) == nil do fiber.sleep(0.01) end
+test_run:wait_upstream(wal_off_id, {status = 'loading', message_re = "Read access to universe"})
 box.cfg { replication = "" }
 
 test_run:cmd("stop server wal_off")