replication: fix flaky gh-3055-election-promote test

Found the following error in our CI:

[001] Test failed! Result content mismatch:
[001] --- replication/gh-3055-election-promote.result	Mon Aug  2 17:52:55 2021
[001] +++ var/rejects/replication/gh-3055-election-promote.reject	Mon Aug  9 10:29:34 2021
[001] @@ -88,7 +88,7 @@
[001]   | ...
[001]  assert(not box.info.ro)
[001]   | ---
[001] - | - true
[001] + | - error: assertion failed!
[001]   | ...
[001]  assert(box.info.election.term > term)
[001]   | ---
[001]

The problem was the same as in recently fixed election_qsync.test
(commit 096a0a7): PROMOTE is written to
WAL asynchronously, and box.ctl.promote() returns earlier than this
happens.

Fix the issue by waiting for the instance to become writeable.

Follow-up #6034

test/replication/gh-3055-election-promote.result

@@ -50,7 +50,7 @@ assert(box.info.election.state == 'leader')
  | ---
  | - true
  | ...
-assert(not box.info.ro)
+test_run:wait_cond(function() return not box.info.ro end)
  | ---
  | - true
  | ...
@@ -86,7 +86,7 @@ assert(box.info.election.state == 'leader')
  | ---
  | - true
  | ...
-assert(not box.info.ro)
+test_run:wait_cond(function() return not box.info.ro end)
  | ---
  | - true
  | ...

test/replication/gh-3055-election-promote.test.lua

@@ -24,7 +24,7 @@ assert(box.info.election.state == 'follower')
 term = box.info.election.term
 box.ctl.promote()
 assert(box.info.election.state == 'leader')
-assert(not box.info.ro)
+test_run:wait_cond(function() return not box.info.ro end)
 assert(box.info.election.term > term)
 
 -- Test promote when there's a live leader.
@@ -35,7 +35,7 @@ assert(box.info.ro)
 assert(box.info.election.leader ~= 0)
 box.ctl.promote()
 assert(box.info.election.state == 'leader')
-assert(not box.info.ro)
+test_run:wait_cond(function() return not box.info.ro end)
 assert(box.info.election.term > term)
 
 -- Cleanup.