Fix more flakiness in ddl_high_priority.inc test

Summary:
This ensures the connection for con2 created in the ddl_high_priority
test is closed before the next test is started.

Reviewed By: Tema

Differential Revision: D6469829

fbshipit-source-id: 556b441

mysql-test/include/ddl_high_priority.inc

@@ -73,6 +73,7 @@ drop table t1_new;
 ## no effect for regular users
 ##
 
+--source include/count_sessions.inc
 connect (con2,localhost,test_user2,,test,,);
 # $con_kill is regular user
 --let $con_kill = con2
@@ -86,6 +87,9 @@ connect (con2,localhost,test_user2,,test,,);
 
 disconnect con2;
 
+# Ensure con2 is closed
+--source include/wait_until_count_sessions.inc
+
 # restore $con_kill
 --let $con_kill = default
 # restore $should_kill