test: fix a typo in election_basic

The typo led to not resetting the election timeout to the default
value. It was left 1000, and as a result the next election tests
could work extremely long.

Part of #5499

test/replication/election_basic.result

@@ -6,7 +6,7 @@ test_run = require('test_run').new()
 -- gh-1146: Raft protocol for automated leader election.
 --
 
-old_election_timeout = box.cfg_election_timeout
+old_election_timeout = box.cfg.election_timeout
  | ---
  | ...
 

test/replication/election_basic.test.lua

@@ -3,7 +3,7 @@ test_run = require('test_run').new()
 -- gh-1146: Raft protocol for automated leader election.
 --
 
-old_election_timeout = box.cfg_election_timeout
+old_election_timeout = box.cfg.election_timeout
 
 -- Election is turned off by default.
 assert(box.cfg.election_mode == 'off')