test_run:wait_lsn() fails with "attempt to compare nil with number"

[Gerold103]

[006] Test failed! Result content mismatch:
[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]   | ...
[006]  
[006]  test_run:switch('default')