The thing is that the following attributes are fixed (remembered) when a trigger
is created:
  - character_set_client
  - character_set_results
  - collation_connection

There are two triggers created in mysql-test/include/mtr_warnings.sql.
They were created using "current default" character set / collation.
is_triggers.test shows definition of these triggers including recorded
character set information.

The problem was that if "current default" changed, the recorded character
set information was not accurate.

There might be two ways to fix that:
  a) update is_triggers.test so that it does not put character-set information
     into result-file;
  b) update mtr_warnings.sql so that the triggers are created using
     hard-coded character sets.

This patch implements option b).