MDEV-21599 plugins.server_audit fails sporadically in buildbot

mysql-test/suite/plugins/t/server_audit.test

@@ -157,6 +157,9 @@ set global server_audit_logging= off;
 set global server_audit_incl_users='root';
 set global server_audit_logging= on;
 disconnect cn1;
+let $count_sessions=1;
+source include/wait_until_count_sessions.inc;
+
 drop user user1@localhost;
 
 set global server_audit_events='';