fix main.mysqld--help-aria failures

when it's run directly after main.mysql_json_mysql_upgrade

because mysqld--help-aria starts a second mysqld that reads the plugin
table, so it has to be flushed and closed at that time.

mysql-test/main/mysqld--help-aria.result

+flush tables;
 #
 # Check that we don't write any data to wrong or not existing datadir
 #

mysql-test/main/mysqld--help-aria.test

 #
 # Check errors from mysqld--help when providing different datadir
 #
+flush tables;
 
 --let $args=--table-cache=5 --max-connections=10 --log-warnings=1 --silent-startup --lower-case-table-names=1 --help --verbose