ci: fix extra region_merge in G13 (pingcap#1148)

close pingcap#1147

Author: wlwilliamx

tests/integration_tests/run_heavy_it_in_ci.sh

@@ -55,7 +55,7 @@ mysql_groups=(
 	# G12
 	'tidb_mysql_test'
 	# G13
-	'fail_over' 'region_merge'
+	'fail_over region_merge'
 	# G14
 	'fail_over_ddl_mix'
 	# G15