MDEV-21857 - Fix sporadic failure of mdev375

status threads_connected can temporarily be bigger than max_connections+1

If SHOW STATUS LIKE "Threads_connected" comes after
ER_CON_COUNT_ERROR is sent to the client, but before the counter is
decremented, Threads_connected can differ from the expected value.

mysql-test/t/mdev375.test

@@ -27,6 +27,8 @@ SELECT 2;
 
 --connection default
 SELECT 0;
+let $count_sessions=11;
+--source include/wait_until_count_sessions.inc
 show status like "Threads_connected";
 
 SET GLOBAL log_warnings=@save_log_warnings;