1) Fixed the test main.audit_plugin_2.

Removed references to SHOW_COMPATIBILITY_56 from test main.audit_plugin_2

2) Improved logging of audit events in performance_schema.global_variables

Moved the call to
  mysql_audit_notify(MYSQL_AUDIT_GLOBAL_VARIABLE_GET)
done in the performance schema to method
  table_global_variables::make_row()
so that audit events are raised only for rows returned to the SQL layer,
instead of generating events for all materialized rows as before.

This makes the following query:
  SELECT * FROM performance_schema.global_variables
     WHERE VARIABLE_NAME = 'foo';
generate only one audit log event,
thanks to the index by variable name on table global_variables.