Add check in wait_lsn for nil value

Found issue in testing:

  [006] --- replication/election_qsync.result	Fri Oct 16 00:12:02 2020
  [006] +++ replication/election_qsync.reject	Sat Oct 17 19:08:11 2020
  [006] @@ -89,6 +89,8 @@
  [006]  -- Wait replication to the other instance.
  [006]  test_run:wait_lsn('default', 'replica')
  [006]   | ---
  [006] + | - error: '...sitories/tarantool/test/var/006_replication/test_run.lua:68: attempt
  [006] + |     to compare nil with number'
  [006]   | ...

Found that wait_lsn() routine from test_run.lua script failed on
comparing returned result from get_lsn() routine with real integer.
It happened because get_lsn() returned 'nil' on the freshly
bootstrapped instance. To avoid of it in wait_lsn() routine added
check that get_lsn() routine returned not 'nil' value.
Complete explanation of the issue and reproducer provided in [1].

This patch does not fix the found issue in tests, but changes the
way it works with the issue. Instead of breaking the test run, for
now it will wait for the lsn value different from nil in loop till
available timeout.

Needed for #226

Co-authored-by: Alexander Turenko <[email protected]>

[1]: #269 (comment)

Author: avtikhon

test_run.lua

@@ -65,9 +65,10 @@ local function wait_lsn(self, waiter, master)
     local sid = self:get_server_id(master)
     local lsn = self:get_lsn(master, sid)
 
-    while self:get_lsn(waiter, sid) < lsn do
+    repeat
         fiber.sleep(0.001)
-    end
+        local waiter_lsn = self:get_lsn(waiter, sid)
+    until waiter_lsn ~= nil and waiter_lsn >= lsn
 end
 
 local function get_vclock(self, node)