plugin_load.test created lines in the warnings files and made
pushbuild cells yellow. Some pushbuild systems are configured
so that they don't build the ha_example plugin. Trying to load
them creates error messages in the error log. These were copied
to the warnings file.

Fixed by ignoring expected error messages for non-existent
ha_example.


mysql-test/lib/mtr_report.pl:
  Bug#35559 - plugin_load.test creates warnings
  Ignore expected error messages for non-existent ha_example.