test: fix for gh_6036 test to avoid flakiness

Promotion of a leader in the test requires previous leader state
arrived to all replicas. Inconsistency may lead to promotion failure,
which is correct from the raft point of view, but not expected by the
test.

Closes #7785

NO_DOC=test
NO_CHANGELOG=test

test/replication-luatest/gh_6036_qsync_order_test.lua

@@ -32,6 +32,8 @@ g.before_all(function(cg)
     })
     cg.cluster:start()
 
+    cg.r1:wait_for_vclock({[1] = 3})
+    cg.r2:wait_for_vclock({[1] = 3})
     cg.r1:exec(function()
         box.ctl.promote()
         box.ctl.wait_rw()
@@ -71,6 +73,7 @@ g.before_test("test_qsync_order", function(cg)
 end)
 
 g.test_qsync_order = function(cg)
+    cg.r3:wait_for_vclock_of(cg.r1)
     cg.r3:exec(function()
         box.ctl.promote()
         box.ctl.wait_rw()