MDEV-10608 - main.mdev375 fails sporadically in buildbot

Test was affected by incompletely closed preceding connections.

Wait for preceding connections to decrement Threads_connected
before testing ER_CON_COUNT_ERROR condition.

Author: svoj

mysql-test/main/mdev375.test

@@ -4,6 +4,8 @@
 --source include/not_embedded.inc
 
 --disable_service_connection
+let $count_sessions=1;
+--source include/wait_until_count_sessions.inc
 set @save_log_warnings=@@log_warnings, @save_max_connections=@@max_connections;
 SET GLOBAL log_warnings=4;
 SET GLOBAL max_connections=10;