WL #1034 (Internal CRON) pre-push updates

- various fixes of test cases
(hopefully the last pre-push update before the push)


mysql-test/include/system_db_struct.inc:
  extend test case to include testing for event system table
mysql-test/r/events.result:
  results of events test
mysql-test/r/system_mysql_db.result:
  more testing - add structure of event system table
mysql-test/t/events.test:
  test case for events (internal CRON)
mysql-test/t/system_mysql_db_fix.test:
  drop event table and leave the test database empty
scripts/mysql_create_system_tables.sh:
  add missing create table statement
scripts/mysql_fix_privilege_tables.sql:
  don't specify the database. this fixes a failing test case which creates the
  tables not on mysql but on a test database and then tests again mysql.
sql/event_executor.cc:
  if asked to stop -> stop.

mysql-test/include/system_db_struct.inc

@@ -12,3 +12,4 @@ show create table tables_priv;
 show create table columns_priv;
 show create table procs_priv;
 show create table proc;
+show create table event;