fix sporadic failures on main.kill

KILL QUERY ID 0 was sometimes finding con3 that was still in the process
of disconnecting and had query_id==0 (as it didn't run any queries)

mysql-test/main/kill.test

@@ -504,6 +504,7 @@ drop table t2;
 --echo #
 --echo # Test kill USER
 --echo #
+--source include/count_sessions.inc
 grant ALL on test.* to test@localhost;
 grant ALL on test.* to test2@localhost;
 connect (con3, localhost, test,,);
@@ -528,6 +529,7 @@ connection con4;
 --error 2013,2006,5014
 select 1;
 connection default;
+--source include/wait_until_count_sessions.inc
 
 --echo #
 --echo # MDEV-4911 - add KILL query id, and add query id information to