replication: fix flaky election_basic test

Found the following error in our CI:

 Test failed! Result content mismatch:
 --- replication/election_basic.result	Fri Aug 13 13:50:26 2021
 +++ /build/usr/src/debug/tarantool-2.9.0.276/test/var/rejects/replication/election_basic.reject	Sat Aug 14 08:14:17 2021
 @@ -116,6 +116,7 @@
   | ...
  box.ctl.demote()
   | ---
 + | - error: box.ctl.demote does not support simultaneous invocations
   | ...
  --

Even though box.ctl.demote() or box.ctl.promote() isn't called above the
failing line, promote() is issued internally once the instance becomes
the leader.

Wait until previous promote is finished
(i.e. box.info.synchro.queue.owner is set)

test/replication/election_basic.result

@@ -95,6 +95,12 @@ test_run:wait_cond(function() return box.info.election.state == 'leader' end)
  | ---
  | - true
  | ...
+test_run:wait_cond(function()\
+    return box.info.synchro.queue.owner == box.info.id\
+end)
+ | ---
+ | - true
+ | ...
 assert(box.info.election.term > term)
  | ---
  | - true

test/replication/election_basic.test.lua

@@ -35,6 +35,9 @@ assert(box.info.election.leader == 0)
 box.cfg{election_timeout = 1000}
 box.cfg{election_mode = 'candidate'}
 test_run:wait_cond(function() return box.info.election.state == 'leader' end)
+test_run:wait_cond(function()\
+    return box.info.synchro.queue.owner == box.info.id\
+end)
 assert(box.info.election.term > term)
 assert(box.info.election.vote == box.info.id)
 assert(box.info.election.leader == box.info.id)